A second explosion, separate from the blast that killed the former Self Defence Force member, caused a fire in a nearby parking lot, while the man's home eight kilometres from the park burnt to the ground, the report said.

Police in the city, about 100 kilometres north of Tokyo, said they had discovered a suicide note written by the man and were investigating the three incidents, but did not give further details.

Images broadcast by NHK showed two burnt-out cars, one of which belonged to the dead man, about 200 metres from the park and a charred broken bench at the edge of the park, where a folk art festival was underway, Reuters reported.

212

The explosion in the park injured two men aged 64 and 58, who were being treated for shrapnel wounds. The third victim was a 14-year-old school boy, NHK said.

Witnesses told the broadcaster they heard a series of loud explosions.

Suicide rates have declined in Japan in recent years, but remain among the highest in the world, with around 30,000 people a year taking their own lives. Experts have pointed to the financial stress of surviving on small pensions for pushing some retired people to end their lives.