Hello Tomohare,
The blue background that you are seeing could be caused by a couple different reasons. However, this issue will require you to request an onsite service call. You can do that by calling Xerox Customer Technical Support for Phaser Products at 1.800.835.6100.
In the future, help can often be found in the Xerox Online Support Assistant here on Xerox.com. The knowledge base articles contain step by step instructions, many with color photos, to help resolve all sorts of problems. I have included a link below to the Phaser 6350 Knowledge Base, should you ever need it.
http://www.support.xerox.com/support/phaser-6300-6350/support/enus.html
