There were 3 stages until it came to be considered as a truth.
1) In 1989, Seiji Yoshida insisted that there was military prostitute hunting in his book.
2) Asashi newspaper reported as if it were a fact about the claim made by Seiji Yoshida.
3) The foreign ministry Youhei Kouno at that time made a comment that sounded like he recognized it as a fact.
About 1), a Korean journalist of the local newspaper in Jejudo island investigated it, they found out that it was total invention.
Also Seiji Yoshida himself recognized that it was a fabrication article in 1996.
About 2), Asahi Newspaper corrected the content of the article in 2014, and they apologized that the article was not the truth. This unusual event has become a big topic in Japan.
About 3) About the comment of the foreign minister, Korean lobby must have worked a lot for this, because he seemed to believe the testimony of the military prostitution without investigating about the evidence at all.
In short, there is no evidence or proof about the military prostitutes except for the comment that the Japanese government made seemed to recognize it as a fact.
In the background where the public opinion was formed into a wrong direction in this way, there seemed to be an accomplishment of agents working abroad.
The name of an agent is not mentioned because he is still alive. He had a code name 'BLYUM' and was inside Asahi Newspaper. In Japanese newspapers, many agents from abroad entered and did covert operations.
We can see how the public opinion was controlled intentionally by looking at such a fact.
In fact, Takuji Yamane, who was ex managing director and Editor-in Chief of Sankei Newspeper, was KGB agent with a code name 'KANT', which is clearly written in Mitrokhin Archive(wikipedia)
.