TOKYO - A man who developed leukaemia after working as a welder at the damaged Fukushima nuclear site is suing the Tokyo Electric Power Company (Tepco) which operates the complex.
The plaintiff, 42, is the first person to be recognised by labour authorities as having an illness linked to clean-up work at the plant, said the BBC.
The nuclear site was hit by the earthquake and tsunami in 2011, causing a triple meltdown.
It was the world's worst nuclear disaster since Chernobyl in 1986. An exclusion zone remains in place around the site as thousands of workers continue clean-up efforts.
The man, from Japan's Fukuoka prefecture, was a welder for a sub-contractor.
He spent six months working at Genkai and Fukushima No 2 nuclear plants before moving to the quake-hit Fukushima No 1 plant, where he build scaffolding for repair work at the No 4 reactor building, the BBC said.
His cumulative radiation exposure was 19.78 millisieverts. This is lower than official limits - Japan currently allows workers at the damaged plant to accumulate a maximum of 100 millisieverts over five years. A dose of 100 millisieverts over a year is seen as enough to raise the risk of cancer.
But in October 2015, a health ministry panel ruled that the man's illness was workplace-related and that he was eligible for compensation, said the broadcaster.
"While the causal link between his exposure to radiation and his illness is unclear, we certified him from the standpoint of worker compensation," a health ministry official said at the time.
The man is now suing Tepco and the Kyushu Electric Power Company (Kepco), which operated the Genkai plant, for 59 million yen (S$740,000).
"I worked there (Fukushima No 1 plant) because of my ardent desire to help bring the disaster under control but I was treated as if I was a mere expendable labourer," Kyodo news agency quoted him as saying.
"I want Tokyo Electric to thoroughly face up to its responsibility."
When he filed the suit late last year, his lawyers said he had been "forced to undergo unnecessary radiation exposure because of the utilities' slipshod on-site radiation management".
Tepco and Kepco have asked the court to reject the suit, questioning the link between his radiation exposure and leukaemia, Kyodo reported.
Tens of thousands of workers have been employed at the Fukushima site since the disaster in March 2011.
Late last year (2016), the government said estimates of clean-up costs had doubled to 21.5 trillion yen.