The fire broke out at the home and office of Koichi Kato in Yamagata prefecture, northern Japan, destroying the buildings, according to local police official Koji Suzuki.
An unidentified man was found collapsed on the premises with wounds to his abdomen. He was rushed to a hospital. There were no other injuries.
Police suspect he may have set fire to the house and then tried to commit suicide.
Mr Suzuki said police were investigating how the fire broke out and that arson “could not be ruled out.”
A member of the ruling Liberal Democratic Party, Mr Kato is known for his criticism of Mr Koizumi’s visits to Yasukuni Shrine, which honours war criminals among the war dead.
Koizumi worshipped at the shrine early yesterday — the anniversary of Japan’s Second World War defeat — drawing praise from some war veterans, rightists and others who believe Japanese leaders should have the right to honour the war dead as they please.
But the visit provoked strong protests from China and South Korea, which suffered heavily under Japanese invasions and view the shrine as a glorification of imperialism.
Mr Kato had made numerous TV appearances yesterday, saying Mr Koizumi should not have visited the shrine only to satisfy his own beliefs.
Mr Koizumi says he goes to Yasukuni to pray for peace and fallen soldiers.
The visit further strained ties with neighbouring nations already driven to their lowest point in decades by Mr Koizumi’s earlier visits to Yasukuni and a host of spats over territory, including natural resources.
Mr Koizumi, however, is scheduled to leave office at the end of September, which means his successor — probably chief cabinet secretary Shinzo Abe — will start with fresh troubles.
“It was a self-righteous decision,” said Yoshinori Murai, a political scientist at Tokyo’s Sophia University. “Japan will be isolated if it shows it doesn’t care about criticisms from home and abroad.”