Akasen (赤線) is Japanese slang and a collective term which was used to identify districts in Japan where prostitution and the sex industry flourished until 1958.
Akasen means literally "red-line". The districts were the designated regions for state-regulated prostitution. Hundreds of government-sanctioned brothels and other adult entertainment parlors were operating in these districts, with the most popular being Yoshiwara in the Akasen region in Tokyo.
Another term Aosen (青線), literally "blue-line", was used for "non-permitted" or "non-legal" districts. In Tokyo, the area directly across the Sumida river from Yoshiwara (Tamanoi, now called Higashi Mukōjima) was a well-established aosen district; it features in some of Kafū Nagai's short stories.
Akasen is often compared directly with the term red-light district in the west.[1] However, this does not explain why the counterpart "non-permitted districts" were called Aosen (blue-line). In practice, the Aosen and Akasen referred to the colors on municipal zoning maps that outlined brothel districts (Akasen) and "normal" entertainment districts (Aosen).
In January, 1946, GHQ issued an order (SCAPIN 642) nationwide to abolish Japan's licensed prostitution system. Commercial brothels continued to operate, but prostitutes were no longer bound by state-guaranteed contracts. All known houses of prostitution were placed on "Off Limits" status by SCAP GHQ which forced many brothels and parlors to change their name to cafē (カフェー) or ryōtei (料亭) in order to continue to attract Occupation business. These businesses fronted a non-adult face (coffee shops, cafes, beer halls, etc.), but would offer sexual services to customers creating a new form of prostitution business. With this development, traditional Akasen prostitution businesses began invading Aosen areas making it difficult for SCAP to identify houses of prostitution and continue its "Off Limits" policy.
In 1958, the Anti-Prostitution Law (売春防止法 baishun-bōshi-hou) was enforced, thus officially abolishing the name Akasen and the districts. However, this did not mean the sex related industry disappeared. To remain in operation, businesses such as soapland (ソープランド sōpurando) and fashion health (ファッションヘルス fashion herusu) are required to file a license application for permission to abide by the sex industry law (風俗営業法 fūzoku-eigyō-hou) in modern days.