diff --git a/doc/source/index.rst b/doc/source/index.rst
index cc408b5..0acb838 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -263,6 +263,8 @@ Notebook
 .. el:variable:: ein:notebook-discard-output-on-save
 .. el:variable:: ein:notebook-modes
 .. el:variable:: ein:notebook-kill-buffer-ask
+.. el:variable:: ein:notebook-querty-timeout-open
+.. el:variable:: ein:notebook-querty-timeout-save
 .. el:variable:: ein:notebook-console-security-dir
 .. el:variable:: ein:notebook-console-executable
 .. el:variable:: ein:notebook-console-args
diff --git a/ein-kernel.el b/ein-kernel.el
index 0782bd0..30e8d6a 100644
--- a/ein-kernel.el
+++ b/ein-kernel.el
@@ -110,7 +110,8 @@ FIXME: document other slots."
 (defun ein:kernel-start (kernel notebook-id)
   "Start kernel of the notebook whose id is NOTEBOOK-ID."
   (unless (ein:$kernel-running kernel)
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'kernel-start (ein:$kernel-kernel-id kernel))
      (concat (ein:url (ein:$kernel-url-or-port kernel)
                       (ein:$kernel-base-url kernel))
              "?" (format "notebook=%s" notebook-id))
@@ -125,7 +126,8 @@ FIXME: document other slots."
   (ein:log 'info "Restarting kernel")
   (when (ein:$kernel-running kernel)
     (ein:kernel-stop-channels kernel)
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'kernel-restart (ein:$kernel-kernel-id kernel))
      (ein:url (ein:$kernel-url-or-port kernel)
               (ein:$kernel-kernel-url kernel)
               "restart")
@@ -391,7 +393,8 @@ http://ipython.org/ipython-doc/dev/development/messaging.html#complete
 (defun ein:kernel-interrupt (kernel)
   (when (ein:$kernel-running kernel)
     (ein:log 'info "Interrupting kernel")
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'kernel-interrupt (ein:$kernel-kernel-id kernel))
      (ein:url (ein:$kernel-url-or-port kernel)
               (ein:$kernel-kernel-url kernel)
               "interrupt")
@@ -403,7 +406,8 @@ http://ipython.org/ipython-doc/dev/development/messaging.html#complete
 
 (defun ein:kernel-kill (kernel &optional callback cbargs)
   (when (ein:$kernel-running kernel)
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'kernel-kill (ein:$kernel-kernel-id kernel))
      (ein:url (ein:$kernel-url-or-port kernel)
               (ein:$kernel-kernel-url kernel))
      :cache nil
diff --git a/ein-notebook.el b/ein-notebook.el
index 6b8831e..c917380 100644
--- a/ein-notebook.el
+++ b/ein-notebook.el
@@ -115,6 +115,28 @@ yet.  So be careful when using EIN functions.  They may change."
     (yes t)
     (t (funcall ein:notebook-discard-output-on-save notebook cell))))
 
+;; As opening/saving notebook treats possibly huge data, define these
+;; timeouts separately:
+
+(defcustom ein:notebook-querty-timeout-open (* 60 1000) ; 1 min
+  "Query timeout for opening notebook.
+If you cannot open large notebook because of timeout error, try
+to increase this value.  Setting this value to `nil' means to use
+global setting.  For global setting and more information, see
+`ein:query-timeout'."
+  :type '(choice (integer :tag "Timeout [ms]" 5000)
+                 (const :tag "Use global setting" nil))
+  :group 'ein)
+
+(defcustom ein:notebook-querty-timeout-save (* 60 1000) ; 1 min
+  "Query timeout for saving notebook.
+Similar to `ein:notebook-querty-timeout-open', but for saving
+notebook.  For global setting and more information, see
+`ein:query-timeout'."
+  :type '(choice (integer :tag "Timeout [ms]" 5000)
+                 (const :tag "Use global setting" nil))
+  :group 'ein)
+
 
 ;;; Class and variable
 
@@ -285,8 +307,10 @@ See `ein:notebook-open' for more information."
   (let ((url (ein:notebook-url-from-url-and-id url-or-port notebook-id))
         (notebook (ein:notebook-new url-or-port notebook-id)))
     (ein:log 'debug "Opening notebook at %s" url)
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'notebook-open url-or-port notebook-id)
      url
+     :timeout ein:notebook-querty-timeout-open
      :parser #'ein:json-read
      :success (cons #'ein:notebook-request-open-callback-with-callback
                     (list notebook callback cbargs)))
@@ -994,8 +1018,12 @@ shared output buffer.  You can open the buffer by the command
     (push `(nbformat . ,(ein:$notebook-nbformat notebook)) data)
     (ein:events-trigger (ein:$notebook-events notebook)
                         'notebook_saving.Notebook)
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'notebook-save
+           (ein:$notebook-url-or-port notebook)
+           (ein:$notebook-notebook-id notebook))
      (ein:notebook-url notebook)
+     :timeout ein:notebook-querty-timeout-save
      :type "PUT"
      :headers '(("Content-Type" . "application/json"))
      :cache nil
@@ -1041,10 +1069,13 @@ shared output buffer.  You can open the buffer by the command
   (ein:events-trigger (ein:$notebook-events notebook)
                       'notebook_saved.Notebook))
 
-(defun ein:notebook-save-notebook-error (notebook &rest ignore)
-  (ein:log 'info "Failed to save notebook!")
-  (ein:events-trigger (ein:$notebook-events notebook)
-                      'notebook_save_failed.Notebook))
+(defun* ein:notebook-save-notebook-error (notebook &key symbol-status
+                                                   &allow-other-keys)
+  (if (eq symbol-status 'user-cancel)
+      (ein:log 'info "Cancel saving notebook.")
+    (ein:log 'info "Failed to save notebook!")
+    (ein:events-trigger (ein:$notebook-events notebook)
+                        'notebook_save_failed.Notebook)))
 
 (defun ein:notebook-rename-command (name)
   "Rename current notebook and save it immediately.
diff --git a/ein-notebooklist.el b/ein-notebooklist.el
index c2554e6..302c20b 100644
--- a/ein-notebooklist.el
+++ b/ein-notebooklist.el
@@ -91,7 +91,8 @@
            (lambda (&rest args)
              (pop-to-buffer
               (apply #'ein:notebooklist-url-retrieve-callback args))))))
-    (ein:query-ajax
+    (ein:query-singleton-ajax
+     (list 'notebooklist-open url-or-port)
      (ein:notebooklist-url url-or-port)
      :cache nil
      :parser #'ein:json-read
@@ -146,7 +147,8 @@
   (assert url-or-port nil
           (concat "URL-OR-PORT is not given and the current buffer "
                   "is not the notebook list buffer."))
-  (ein:query-ajax
+  (ein:query-singleton-ajax
+   (list 'notebooklist-new-notebook url-or-port)
    (ein:notebooklist-new-url url-or-port)
    :parser (lambda ()
              (ein:notebooklist-get-data-in-body-tag "data-notebook-id"))
@@ -200,7 +202,9 @@ This value is used from `ein:notebooklist-new-scratch-notebook'."
 
 (defun ein:notebooklist-delete-notebook (notebook-id name)
   (message "Deleting notebook %s..." name)
-  (ein:query-ajax
+  (ein:query-singleton-ajax
+   (list 'notebooklist-delete-notebook
+         (ein:$notebooklist-url-or-port ein:notebooklist) notebook-id)
    (ein:notebook-url-from-url-and-id
     (ein:$notebooklist-url-or-port ein:notebooklist)
     notebook-id)
diff --git a/ein-query.el b/ein-query.el
index e0267ef..f98effc 100644
--- a/ein-query.el
+++ b/ein-query.el
@@ -46,8 +46,24 @@
 
 ;;; Variables
 
-(defcustom ein:query-timeout 5000
-  "Default query timeout for HTTP access in millisecond."
+(defcustom ein:query-timeout 1000
+  "Default query timeout for HTTP access in millisecond.
+
+Setting this to `nil' means no timeout.
+
+If you do the same operation before the timeout, old operation
+will be canceled \(see also `ein:query-singleton-ajax').
+
+.. note:: This value exists because it looks like `url-retrieve'
+   occasionally fails to finish \(start?) querying.  Timeout is
+   used to let user notice that their operation is not finished.
+   It also prevent opening a lot of useless process buffers.
+   You will see them when closing Emacs if there is no timeout.
+
+   If you know how to fix the problem with `url-retrieve', please
+   let me know or send pull request at github!
+   \(Related bug report in Emacs bug tracker:
+   http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11469)"
   :type '(choice (integer :tag "Timeout [ms]" 5000)
                  (const :tag "No timeout" nil))
   :group 'ein)
@@ -199,7 +215,7 @@ is killed immediately after the execution of this function.
 (defun* ein:query-ajax-timeout-callback (buffer &key
                                                 (error nil)
                                                 &allow-other-keys)
-  (ein:log 'debug "EIN:QUERY-AJAX-TIMEOUT-CALLBACK buffer = %s" buffer)
+  (ein:log 'debug "EIN:QUERY-AJAX-TIMEOUT-CALLBACK buffer = %S" buffer)
   (ein:with-live-buffer buffer
     (setq ein:query-ajax-canceled 'timeout)
     (let ((proc (get-buffer-process buffer)))
@@ -212,6 +228,31 @@ is killed immediately after the execution of this function.
     (cancel-timer ein:query-ajax-timer)
     (setq ein:query-ajax-timer nil)))
 
+(defvar ein:query-running-process-table (make-hash-table :test 'equal))
+
+(defun ein:query-singleton-ajax (key &rest args)
+  "Cancel the old process if there is a process associated with
+KEY, then call `ein:query-ajax' with ARGS.  KEY is compared by
+`equal'."
+  (ein:query-gc-running-process-table)
+  (ein:aif (gethash key ein:query-running-process-table)
+      (ein:with-live-buffer it
+        (setq ein:query-ajax-canceled 'user-cancel)
+        (let ((proc (get-buffer-process it)))
+          ;; This will call `ein:query-ajax-callback'.
+          (delete-process proc))))
+  (let ((buffer (apply #'ein:query-ajax args)))
+    (puthash key buffer ein:query-running-process-table)
+    buffer))
+
+(defun ein:query-gc-running-process-table ()
+  "Garbage collect dead processes in `ein:query-running-process-table'."
+  (maphash
+   (lambda (key buffer)
+     (unless (buffer-live-p buffer)
+       (remhash key ein:query-running-process-table)))
+   ein:query-running-process-table))
+
 (provide 'ein-query)
 
 ;;; ein-query.el ends here