A man who developed leukemia after working on a cleanup crew at the ruined Fukushima Daiichi nuclear power plant has been awarded workers’ compensation by the Japanese government, in what amounts to the first official acknowledgment that exposure to radiation at the disaster site may have caused cancer.

The Ministry of Health, Labor and Welfare said on Tuesday that the man, whom it did not identify by name, worked from October 2012 to December 2013 installing protective covers over damaged reactor buildings at the site.

More than 40,000 workers have participated in the vast and laborious cleanup effort at Fukushima Daiichi since the plant was struck four and a half years ago by a tsunami, leading to multiple reactor meltdowns and blanketing the area with toxic debris. A number of those workers have since been found to have cancer and are seeking compensation from public insurance programs or the plant’s owner, the Tokyo Electric Power Company, known as Tepco.

The compensation awarded to the man would cover treatment costs by supplementing his standard national health insurance.

Their claims are both politically and medically fraught. Pinpointing the cause of any individual cancer is usually impossible and, with so many workers involved, some cases would inevitably arise regardless of radiation exposure. About one in 150 Japanese receives a cancer diagnosis each year, according to the country’s National Cancer Center. The disease is by far the leading cause of death in Japan.

Questions about the Fukushima accident’s health effects have fueled a polarizing national debate. About 80,000 former residents remain displaced from the vicinity of the plant, and although the government is gradually rolling back the evacuation zone and spending $10 billion to scrape up contaminated soil, many say they do not want to return. Radiation levels are well below what most scientists say are dangerous, but they are still measurably higher than they were before the disaster — a confounding signal for people making consequential decisions about their lives.

Differences among experts have added to the uncertainty: One well-publicized recent study found an elevated incidence of thyroid cancer in children living near the plant, for example, but its results have been contradicted by other research.

Plant workers face a different magnitude of risk. Tepco has been criticized for the sometimes sloppy training and protection it has offered cleanup workers at Fukushima, many of whom are unskilled and hired through arm’s-length subcontractors and labor brokers. A government examination of the utility’s safety practices in 2013 found that it had underreported the radiation exposure of a third of the workers whose records were reviewed by inspectors.

The worker who developed leukemia was exposed to less radiation than many, according to the Health Ministry, suggesting that the number of compensation cases could balloon. Nearly half the workers who have spent time on Fukushima Daiichi cleanup crews, or more than 20,000 people, have been exposed to enough radiation that subsequent cancers could qualify as occupational illnesses, the ministry said.

The ministry said the worker was exposed to 15.7 millisieverts of radiation during his 14 months at Fukushima Daiichi. That was three times the level of five millisieverts a year required to claim workers’ compensation, though it was still well below the maximum of 50 millisieverts that safety regulators allow for nuclear workers. Before joining the Fukushima cleanup, he had also worked for several months at another nuclear plant, where he had absorbed an additional, though smaller, dose.

Shigenobu Nagataki, an expert at Nagasaki University on radiation exposure, said the likelihood that there was a direct link between the man’s work and his leukemia was low, given his reported dose level. “But it’s still necessary to pay close attention to workers’ exposure,” Mr. Nagataki told the public broadcaster NHK.

The Health Ministry said that as of Tuesday, eight Fukushima cleanup workers had applied for compensation through a government-supervised occupational health insurance system. One claim was withdrawn, three were rejected and three are still pending, it said.