ckedittor automcalliy convert < When I copy a link and put into CKeditor (drupal)

&lt;iframe width="420" height="315" src="//www.youtube.com/embed/q4Rgh5JNWf4" frameborder="0" allowfullscreen&gt;&lt;/iframe&gt;

After I switch back plain text it changed to be like this:

&lt;p&gt;&amp;lt;iframe width="420" height="315" src="//www.youtube.com/embed/q4Rgh5JNWf4" frameborder="0" allowfullscreen&amp;gt;&amp;lt;/iframe&amp;gt;&lt;/p&gt;

How can I solve this?
First put your text format to Plain Text mode and then paste your &lt;iframe&gt; code on the editor, now you switch back to "Full HTML" or "Filtered HTML" mode (for which your text editor is set to display).

Hope this is what you are looking for. Hope this helps. :)