Armature completely destroys model upon moving bone Good day. 
I'm completely new to Blender. And I really want to get along with it, but... it's quite more complex than I thought. 

Let a example talk for me.



I made this model. A rather simple model. And I'm trying to give it an armature. I added the bones, and whatnot. However, when I set the mesh parent to the armature, and move it with Pose Mode, the armature completely obliterates the mesh.

What am I doing wrong? Could you guys please explain this to me?
I'm still quite new to Blender, but this issue happens constantly. 

Update: 
This is the .blend file of my model.
It is because automatic weights not always works good. Sometimes, especially if you have multi-part mesh you need to edit vertex groups manually:



until more suitable result