Mineo is a Japanese MVNO that provides SIM cards with a choice of the AU, NTT DoCoMo or SoftBank networks.
They have a range of different contract data plans, starting at 500MB/month and going up to 30GB. Voice calling is also a possibility, but costs extra. Data-only prepaid SIM cards are also available.
Note: The level of English-language support available from Mineo is unclear. Also, the website, which is in Japanese, may not be accessible to overseas visitors.
See how Mineo compares to other prepaid or long-term mobile plans in Japan.