A prepaid SIM is a SIM card you pay for in advance and then use for a fixed period or a fixed amount of data. There is no ongoing monthly contract: the line stops once the allowance is used up, and some products let you top it up and carry on. It suits travelers, short-stay visitors, and anyone who wants a secondary line just to stay reachable.
In Japan, prepaid SIMs are sold at airport vending machines, convenience stores, and electronics retailers. Products aimed at overseas visitors are mostly data-only, run for anything from a few days to about a month, and as of 2026 sit within a price band of a few thousand yen. Because the data allowance comes bundled with the period, choose by allowance if you watch a lot of video and by period if you only need to stay contactable. To take out a line that can make voice calls, you must have your name, address, and date of birth verified under Japanese law on preventing improper use of mobile voice communications services; the same law also allows a retailer or other intermediary to carry out that check on the carrier's behalf.
What deserves a check before you buy is the list of things the SIM cannot do. A data-only SIM, or a calling app that works through an 050 number, cannot reach emergency services on 110 or 119. Some SIMs handle no SMS either, in which case you cannot receive the one-time passwords that hotel bookings and payment services send. Where a product has a fixed validity period, any data left over becomes unusable once that period ends, so work the period out backwards from your travel dates.
On the criminal side, lines whose registered holder is not the person actually using them show up in special fraud as so-called burner phones, because a line intended to be used up quickly suits anyone trying to avoid being traced. Japanese law on preventing improper use of mobile voice communications services therefore requires carriers to verify identity not only when a contract is signed, but also when a line or SIM is passed to someone else with a change of registered holder. See also managing phone number privacy.