The man was carrying equipment at the Fukushima Dai-ichi nuclear plant when he collapsed. He died later in hospital, said Tokyo Electric Power Co. spokesman Naoyuki Matsumoto.
The company does not know the cause of his death, Matsumoto said. The man had begun working Friday at the waste disposal building, which stores radioactive-contaminated water that has leaked from the plant's tsunami-crippled reactors.
He had been wearing a radiation protection suit, mask and gloves, and no radioactivity at harmful levels was detected in his body, the spokesman said.
The Fukushima plant lost its power and cooling systems in the March 11 earthquake and tsunami, triggering fires, explosions and radiation leaks in the world's worst nuclear accident since Chernobyl in 1986.
