DMM Mobile is a Japanese MVNO that offers a wide range of data plans for mobile devices, on a long-term contract basis. Minimum contract length is a year.
You can choose from data-only or data+voice DMM Mobile SIM plans. Data packages run from 1GB (or less) all the way up to 20GB. Sharing with family is a possibility. If you need a new phone, you can add one to your contract—various handsets are available.
DMM Mobile Wifi is another product available from the provider. This provides portable wifi on a long-term, contract basis.
Note: The DMM Mobile website is in Japanese, and the level of English-language support available is unclear. Intermediate or advanced Japanese-language skills are recommended.