I thought that it is a little bit complicated to assign an app to Spaces using System Preferences.
Today I googled to create an apple script to do it easily.
After some googling, I just found one good script here, “Applescript Forums | MacScripter / Spaces Library”.
It is awesome and perfect for this aim.
1. Copy, paste and save the script to a local file as an apple script.
2. Put it to the User Script Folder (~/Library/Scripts/).
3. Put the Script Menu on the Menu Bar using AppleScript Utility.app.
Now you can assign the forefront app to any Spaces easily.
Spaces Library
For Japanese environment, you have to modify 3 lines in the script.