Tokyo Electric Power Co., operator of the Fukushima Daiichi plant, which was struck by a March 11 earthquake and tsunami, said the man had been exposed to a small 0.5 millisievert of radiation but showed no internal exposure to radiation, Kyodo News said.
Millisieverts measure biological damage caused by radiation.
Doctors said the man, reportedly in his 40s, died of acute leukemia.
Tokyo Electric, citing results of examinations by doctors, said the man's death was not connected to his work at the plant.
The plant is yet to be brought under control following the nuclear crisis caused by the earthquake and tsunami.