Can my friend use my bike while I am away?

I am about to go back to my home country for about a month and a half, and I need some place to keep my bike. I figured that I'd just let my friend use it for that period, but I am concerned that he'll face trouble from the police if they find that the bike is not registered to him.
Is there any way around this?

In term of registration process, if you show in a written statement(in Japanese) that the bicycle is your own property but your friend can use it while you are away(eg, between March 2017 - April 2017), with your full name, address, phone numbers, registration numbers(Bouhan-toroku)and your signature. Your friend must hold the written statement always as he/she rides on the bicycle.