The abuses by Johnny Kitagawa, the man behind some of the most popular Japanese boy bands such as SMAP and Arashi, had been reported by local tabloid Shukan Bunshun as far back as 1999. Kitagawa sued the magazine, but major media outlets steered clear of covering the lawsuit or chasing the story.
It was only after Britain's BBC aired a tell-all documentary in March - four years after Kitagawa's death at the age of 87 - that local mass-media followed suit, stirring national outrage.
"This issue had been reported frequently in weekly magazines and other publications, and the Tokyo High Court's ruling on the fact of sexual assault was confirmed in 2004," public broadcaster NHK said in a statement on Thursday.
"But NHK's awareness over this issue was wanting, and we never followed up with deeper reporting or chose to take it up," it said, promising improvements.
A third-party committee set up to investigate the allegations reported last week that Kitagawa had sexually abused hundreds of boys and young men, and faulted the media for its silence.
Kauan Okamoto, a victim of Kitagawa's abuses, has said he had not been aware of allegations prior to joining the agency because they were scarcely reported.
Other major broadcasters including NTV, TV Asahi and Fuji TV, also issued statements promising to do better.
"The conduct of the media on the issue of sexual assault is also being questioned (with this case)," TV Asahi said. "We will heed the voices of victims, the (third-party team's) report, and the opinions and suggestions of viewers seriously, and reflect them in our future broadcasts and activities."
(Reporting by Francis Tang and Maki Shiraki; Editing by Chang-Ran Kim and Mark Heinrich)
By Francis Tang and Maki Shiraki
