Hi. If you’re only in the country for up to a month then you may find it best to go with one of the straightforward tourist packs. All of these piggyback off the main carriers. Of these, NTT Docomo, has the best network coverage. These are available at the airports or in electronic or phone stores. Note that identical SIMs cost more at the airport than in town.
JP-mobile offers a standard voice-SMS-data package and is valid for 15 days only. You can buy it at the airports. It is markedly more expensive than data-only options.
The Japan Travel SIM comes with a Bratsel VOIP card, which permits international calls and also gives you a local phone number for receiving calls. It’s only valid for up to one month and is non-extendable. It can be purchased at airports or in electronics shops.
If you’re in Japan for a longer period some of the companies offer extendable SIMs (data only). We’ve tried COMST and So-Net. Both run off NTT Docomo and have excellent coverage. Neither throttles their data. For So-Net you need a credit card to top-up online within 14 days of your SIM expiring. With COMST no credit card is necessary; you top-up online within one month of your card expiring and pay at a convenience store. So-Net is easy to get hold of being widely sold at the airports and electronics stores and even in-flight (prices for the same card vary wildly though).