An area code is a regional identification number assigned to the beginning of a landline phone number. Under the Ministry of Internal Affairs and Communications' Telecommunications Numbering Plan, area codes are systematically assigned to municipalities across Japan. Since you can estimate the caller's region just by looking at the number, area codes are also useful for phone number lookups and identifying nuisance calls.
Area codes range from 2 to 5 digits, with larger cities receiving shorter codes. 03 (Tokyo's 23 wards) and 06 (Osaka City) are 2 digits, 045 (Yokohama) and 011 (Sapporo) are 3 digits, and 0166 (Asahikawa) is 4 digits. Shorter area codes leave more digits available for the local exchange number and subscriber number, allowing more phone numbers to be accommodated.
A commonly confused concept is the prefix, but area codes are a term limited to regional identification for landline phones. Mobile phone numbers starting with 090/080/070 and IP phone numbers starting with 050 are not area codes - they are managed under a separate telecommunications numbering system. Knowing how to look up area codes helps you quickly identify the region when receiving calls from unknown numbers.
An important practical note: even within the same area code, local call rates may not always apply. For example, within the 03 area, calls crossing ward boundaries may be treated as adjacent-zone calls, so it is important to properly understand phone number structure. Even after NTT's migration from PSTN to an IP network (completed in 2024), the area code system has been maintained, and the way users dial numbers remains unchanged. However, area codes may be consolidated or reorganized due to municipal mergers or demographic changes, so it is wise to check the latest mappings on the area code map. You can also review the steps for identifying regions from numbers in the area code lookup guide.