NHK has learned that two employees of Nippon Steel were exposed to radiation doses far above the legal limit while handling an X-ray device.
The company and police say that the two men inspected the device at a factory in the western Japanese city of Himeji in Hyogo Prefecture on May 29. They complained of ill health the next day.
The employees, in their 50s and 30s, are being treated at a medical facility in Hiroshima University that provides advanced medical care for radiation exposure.
It isn't known how high the doses were. Sources told NHK that the men may have been exposed to radiation of up to several dozen times the annual limit of 50 millisieverts. This limit applies to workers who handle radiation.
Nippon Steel says the factory produces iron plates for automobiles. It says the employees were inspecting the X-ray device to measure the thickness of the coating on the plates' surfaces.
The steelmaker adds that the accident did not cause radiation to leak outside the plant. It has not disclosed the conditions of the workers, citing their privacy. The firm promised to cooperate with the authorities' inquiry and said it would make efforts to prevent recurrence of a similar accident.
A labor standards inspection office and police are looking into the cause of the accident.
The labor ministry suspects that the device mistakenly kept emitting X-rays during the inspection.
They issued a notice dated June 1 to urge businesses that use similar devices to ensure that employees won't be exposed to excessive doses of radiation.