The blasts took place at a park in Utsunomiya, some 100 kilometres (60 miles) north of Tokyo.
Three explosions took place in a Japanese park on Sunday, killing one person and injuring at least three, state broadcaster NHK has reported.
The blasts occurred at a park in Utsunomiya, some 100 kilometres (60 miles) north of Tokyo, shortly after 11:30am (02:30 GMT), a fire department spokesman said. The cause of the blasts is currently unknown.
"One person was found dead," the fire department spokesman told AFP, without elaborating further.
"The gender and ages of the three injured are not known yet," another local fire department official said, adding one blast hit a car parking lot for the park.
Images broadcast by NHK showed two burnt-out cars in a parking lot about 200 metres (650 feet) from the park, and a charred broken bench at the edge of the park where a folk art festival was underway.
Explosions of this kind are rare in Japan, although small pipe bombs blasts linked to extreme leftist group occasionally take place near US military bases.
In November last year a homemade pipe bomb exploded at the controversial Yasukuni war shrine in Tokyo. The attack resulted in damage to toilets at the facility, but no one was hurt.
A South Korean man was later arrested and sentenced to four years in prison after admitting to detonating the bomb at the shrine, which has been targeted by activists who see it as a symbol of Japan's militaristic past.