An area code map is a visual representation on a map showing which area codes are assigned to which regions across Japan. It allows intuitive understanding of number distribution patterns and is useful for quickly estimating the origin of calls from unknown numbers.
Japan's area codes were systematized during the nationwide automatic dialing rollout (establishing direct-dial capability from anywhere in the country) in the 1960s. As a general rule, numbers increase from north to south. 01 covers Hokkaido, 02 covers Tohoku and northern Kanto, 03 is Tokyo's 23 wards, 04 covers southern Kanto and parts of Koshinetsu, 05 covers Chubu and Tokai, 06 is Osaka city, 07 covers parts of Kinki, Chugoku, and Shikoku, 08 covers parts of Chugoku, Shikoku, and Kyushu, and 09 covers Kyushu and Okinawa. This system was designed to optimize relay routes between telephone exchanges, with geographic proximity roughly corresponding to numerical proximity.
However, numerous exceptions exist due to historical circumstances. For example, 04 is also assigned to Kashiwa and Abiko cities in Chiba Prefecture, so "04 = Kanagawa" is not always true. These exceptions result from municipal mergers and population growth triggering number block reorganization. Also, among numbers starting with 0 as a prefix, 050 (IP phone), 070/080/090 (mobile phone), 0120 (toll-free), and 0570 (Navi-Dial) are not tied to geographic regions and are not included in area code maps.
A practical use of area code maps is estimating the origin of suspicious calls. For example, a number starting with 06 suggests a call from the Osaka area, and if you have no connection to that region, you can suspect a nuisance call. However, if caller ID spoofing is used, the displayed number and actual origin may differ, so overreliance is unwise. See How to Look Up Area Codes for specific steps to identify regions from numbers and Phone Number Structure Guide for the overall numbering system.