MNP (Mobile Number Portability) is a system that lets you switch carriers while keeping your mobile phone number. Introduced in Japan in October 2006, it did much to encourage competition between carriers. Before it existed, the trouble of changing numbers was the single biggest barrier to switching; MNP let users keep their number and choose freely on the basis of price and service.
Number portability and MNP describe the same idea, but MNP is the term reserved for mobile phones. The fixed-line equivalent is called LNP (Local Number Portability), and it comes into play when moving to a service such as Hikari Denwa. Being able to choose a provider without giving up your number also became a precondition for later entrants such as MVNOs (budget SIMs) to win over existing subscribers.
Two MNP procedures exist side by side. In the traditional method, you obtain an MNP reservation number (valid for 15 days) from your current carrier and use it when signing up with the new one. In addition, for online applications where both the old and the new carrier support it, a one-stop method lets you complete the whole transfer by applying on the new carrier's website, with no need to contact the outgoing carrier. With an eSIM-capable handset there is no need to visit a shop at all, and in some cases the line can be in use the same day you apply. According to guidance from the Ministry of Internal Affairs and Communications, transferring out is free of charge online, while a fee may apply by phone or in a shop (as of August 2026). How the receiving carrier handles fees varies by contract, so check each company's guidance before you apply.
One caveat: carrying your number across does not carry your carrier email address (@docomo.ne.jp, @ezweb.ne.jp, @softbank.ne.jp) with it. Some carriers offer a service that lets you keep the address, but the conditions and the monthly charge differ from carrier to carrier. If switching is a possibility, giving friends and online services an address that does not depend on your carrier saves you from having to announce a change every time you move. Depending on when the SIM card is switched over, there may also be a short window in which calls and data are unavailable. Review the latest procedures in the MNP procedure guide.
MNP was introduced against a background in which a fair number of users gave up on switching simply because they did not want their number to change. Guaranteeing number continuity as a matter of policy moved the decision to switch onto price and service content instead. The other thing to check is the handset's SIM lock: depending on when and where it was bought, a handset may still need to be SIM-unlocked, so confirm that it will work on the line you are moving to.