It’s possible to buy pay-as-you-go SIM cards directly from Japan’s major mobile providers at airports in Japan, but you can usually get a better deal with an MVNO. This stands for Mobile Virtual Network Operator; they piggyback on existing big networks to offer some great deals and good data communication.

Where should I buy prepaid SIM Card?

The cheapest way to get a prepaid SIM card is to order it online and pick it up at the airport or at your accommodation. Prepaid SIMs sold at airports tend to be more expensive in general. There are large Electronic Stores in Tokyo and Osaka where you can buy prepaid SIM cards, but they may not have enough stock, so it is better to order a prepaid SIM card online in advance. Where to Buy A Prepaid SIM Card in Japan

NTTDoCoMo has the largest network coverage, and almost all of the prepaid SIM card services in Japan are based on their network.
NTTDoCoMo offers 4G (LTE) network under the service name of Xi, having a wide range of frequency bands: 700, 800, 1500, 1800 and 2100 MHz.
Some of the frequency bands are also used in countries other than Japan, however, the 1800 MHz, one of the most usual bands in the world, is available only in the following major cities: Nagoya, Tokyo, and Osaka.
As you have seen above, a usable frequency can vary from place to place, so, its highly recommended to check which band do you need for the area you want to go and which band is your device compatible with before you actually visit Japan.

Also, you have to make sure that your device must be unlocked (sim lock free)
and must support one of the following frequencies: