A SIM card (Subscriber Identity Module) is a small IC card inserted into mobile phones and smartphones. It stores subscriber identification information (IMSI: International Mobile Subscriber Identity), phone numbers, and authentication keys, and is required to connect to a carrier's network. Without a SIM card, a device cannot make calls or use data (except via Wi-Fi).
SIM cards have shrunk over three generations: standard SIM (25mm x 15mm, the 2G/3G era), micro SIM (15mm x 12mm, adopted with the iPhone 4), and nano SIM (12.3mm x 8.8mm, adopted from the iPhone 5 onward). On handsets that take a physical card, nano SIM is the standard size. The alternative is the eSIM, which writes the line profile to a chip inside the handset rather than to a card you insert, and some models have no physical SIM slot at all. The iPhone 16 and iPhone 16 Plus sold in the United States, for example, are eSIM-only: Apple's specification sheet states that they are not compatible with physical SIM cards (as of August 2026). Because the same model can be sold with different SIM arrangements depending on the country, always check the SIM line of the specification sheet when buying a handset abroad.
Moving a SIM card to another handset lets you use the same phone number on a different device. Swapping in a local SIM card while travelling to avoid steep roaming charges is common practice. In Japan, newly sold handsets are as a rule not SIM-locked (a restriction that blocks other carriers' SIM cards). Some handsets still need to be unlocked, however, depending on when and where they were bought, so if you are using a second-hand device, check the supported frequency bands and ask your provider whether unlocking is possible and on what terms.
The main security concern around SIM cards is the SIM swap attack, in which an attacker deceives a carrier into moving the victim's number onto a different SIM and can then intercept two-factor authentication codes sent by SMS. If a SIM card is lost or stolen, contact the carrier immediately to suspend the line. Note also that when a mobile contract involving a SIM is signed, and when a contract is transferred to a new holder, identity verification is carried out under Japan's Act on Prevention of Improper Use of Mobile Voice Communications Services (Articles 3 and 5). Not casually taking over or lending out a SIM registered in someone else's name is the basic precaution against being drawn into SIM-based crime. Review SIM-related procedures in the number portability guide.