Read this account by Mathieu Gaulène, who addresses a Japanese-language book by Honma Ryu claiming that censorship was exercised by Japan's largest communication conglomerate, Dentsu:
Mathieu Gaulène, M. (2016). Does the advertising giant Dentsu pull the strings of the Japanese media? Asia Pacific Journal: Japan Focus, 14(11.5), http://apjjf.org/2016/11/Gaulene.html
Dentsu, the fifth largest communication group in the world, holds a large share of the Japanese advertising market, which impacts media freedom in Japan. This is particularly true in relation to the nuclear power industry.
Dentsu, the 5th communication group in the world and the number one ad agency. With its rival Hakuhodo, 2nd in the archipelago, the two agencies nicknamed “Denpaku,” combine advertising, public relations, media monitoring, crisis management for the largest Japanese and foreign companies, the local authorities, political parties or the government. Together they hold nearly 70% of the market. A true empire that some accuse of ruling the roost in the Japanese media…
… In a book published in 2012, Honma Ryu looked into some of Dentsu's backstage, and its tight control over the media, especially on behalf of one of its major clients: Tokyo Electric Power Company, Tepco. Honma is not alien to advertising circles; he worked for 18 years at number 2, Hakuhodo, then after one year imprisonment for fraud, he began writing, first about his prison experience, then about his years of advertising and the methods he used to coax the media. In 2012, his book Dentsu and Nuclear Coverage became a bestseller within a few months, despite almost universal media blackout.
…Honma meticulously described the mechanisms by which Dentsu, the inevitable intermediary, implicitly imposes on media what can or cannot be written on nuclear power, and under what conditions. “Dentsu occupies a special position since the agency holds 80% of the market for nuclear advertising in Japan,” he reminded us during an interview in a coffee shop at Ueno Station....
I strongly recommend reading entire article at link above.