In addition, he was a member of the House of Representatives for the Social Democratic Party until July 21, 2009.

Hosaka campaigned and won the mayor's job on an anti-nuclear platform in April 2011, just over a month after the Fukushima Daiichi nuclear disaster.

According to The Wall Street Journal, Hosaka "is determined to turn this city ward of 840,000 people, the largest in Tokyo, into the front-runner of a movement that will put an end to Japan's reliance on atomic power and accelerate the use of renewable energy".

[5] He is also highly critical of continued Japanese moral panic against the Otaku subculture in the nation, with one notable case of criticism displayed against public and media related assumptions and gossip surrounding the incidents of November 17, 2004, when Kaoru Kobayashi murdered a 7-year-old girl.

Mizuho Fukushima, then-leader of the Social Democratic Party, also joined the event with him in 2007.