Yes, 360travelsim works in both countries. To use it in Japan you will need a 3GSM (or 3G) mobile phone that supports WCDMA 2100. Although this is a GSM phone it is using 3rd generation technology which is deployed in Japan.
There are 2 different 3G standards presently in use. To ensure that you have the correct phone for roaming in Japan, it is advisable that you check with either your current network operator, or refer to their mobile’s handbook to determine whether or not it is WCDMA 2100 compatible.