Are you artistic? I am not so this isn't for me. But I found this great website where you can have your own designs printed onto t-shirts. Then you can sell them online via their site. How easy is that?
You can visit the US site to get the idea but they also have a Japanese website.
I copied this from there.
It's so easy!
1. Register for free
2. Upload your designs
3. Add designs to the products
4. Set a selling price/your profit
5. Publish your shoplink or subdomain - You're done
Have a look at SHIRTCITY
T Shirts are so hot now. Vintage styles, cartoon characters, environmental statements , use your imagination! Another great business idea and totally do-able from Japan.