Former Prime Minister Junichiro Koizumi criticized the government's push to restart nuclear power generation and urged it to reconsider.
"It is irresponsible to pursue nuclear power when there is no permanent disposal site for radioactive waste," Koizumi said at a lecture in Nagoya.
The government under Prime Minister Shinzo Abe has retracted its predecessor's goal of phasing out nuclear power and opened the way for the restart of idled reactors.
The former prime minister, who appointed Abe as his chief Cabinet secretary, said that during his time as a Lower House lawmaker, he believed expert opinion that nuclear energy is environmentally friendly and the cheapest source of power.
But the March 2011 earthquake and tsunami, and subsequent nuclear crisis, changed his mind. "I started having doubts about whether (nuclear power) is really safe and cheap," he said.
"Japan can still stand even without nuclear power plants," Koizumi said, calling on the government and Liberal Democratic Party to take the initiative in pursuing a policy that does not depend on nuclear plants.
Koizumi said that after visiting Germany and seeing the way the country is pursuing renewable energy, he changed his mind and wants Japan to make use of natural energy sources.
Currently, all of Japan's 50 commercial reactors are offline and have to be checked by the Nuclear Regulation Authority to determine whether they satisfy a set of new safety requirements before they can be restarted.