in order to buy japanese book in kobo store
1. you have to setup a rakuten.co.jp account,
2. setup vpn and access rakuten.kobobooks.com with japan IP address
3. choose login by rakuten id
once success, you don't need to setup vpn to buy book in the future
don't bother to do this if you can't read japanese
free vpn service
http://www.vpngate.net/