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 card sizes have shrunk over three generations. Standard SIM (25mm x 15mm, 2G/3G era), micro SIM (15mm x 12mm, adopted with iPhone 4), and nano SIM (12.3mm x 8.8mm, adopted from iPhone 5 onward). As of 2024, nano SIM is the mainstream for smartphones. eSIM, which requires no physical card, is also rapidly spreading, with iPhone 14 (US model) and later eliminating the physical SIM slot entirely. In the future, iSIM integrating SIM functionality into the SoC (processor) is also expected.
By swapping a SIM card to a different device, you can use the same phone number on different handsets. Swapping to a local SIM card when traveling abroad to avoid expensive roaming charges is also common practice. In Japan, SIM locking (restricting use of other carriers' SIM cards) on newly sold devices has been prohibited in principle since October 2021. Devices purchased before that date can also be SIM-unlocked for free through each carrier's website.
A security concern with SIM cards is SIM swap attacks. Attackers trick carriers into transferring a victim's number to a different SIM, enabling interception of two-factor authentication SMS codes. If your SIM card is lost or stolen, immediately contact your carrier to suspend the line. Review SIM-related procedures in the number portability guide.