Japanese health authorities set the permissible cumulative level for radiation exposure for workers at nuclear power plants at 50 millisieverts per year to protect their safety.
If their exposure exceeds the limit, they are not allowed to work inside radiation control areas.
Employers violating the rule can be punished with prison terms of up to 6 months or fines of up to 500,000 yen or about 6,370 dollars.
Worker exposure is measured by dosimeters they wear while at work, and their employer is responsible for managing them.
At the Fukushima plant, workers receive dosimeters before starting work and return them after work.
The plant's operator, Tokyo Electric Power Company, and one of its contractors, Tokyo Energy & Systems, monitored the exposure of workers from Buildup, a subcontractor with an executive suspected of ordering adjustment of dose records. Workers were informed of their doses every day, and cumulative figures were reported to Buildup once a month.
Such data is also reported to the Radiation Effects Association on a regular basis to keep track of workers' cumulative doses even if their employers change.
Buildup's workers were carrying a dosimeter called "Glass Badge" in addition to the digital one they were told to cover with lead.
The health ministry plans to compare the data on both dosimeters for any significant differences, and also compare data records of people working at the same site.
Build-up President Takashi Wada says he's aware of the gravity of the issue.
The president of another subcontractor told NHK that work in a highly radioactive environment is lucrative.
But he said it's out of the question to falsify exposure data. He said he fears the revelation could undermine confidence in nuclear subcontractors.
A man who had worked at the Fukushima plant just after last year's accident says the official dose limit is 50 millisieverts per year but in reality contractors avoid sending workers to the site if the reading goes beyond 20 millisieverts.
He says he himself can no longer work at nuclear power plants as his cumulative dose probably exceeds about 30 millisieverts.
The man says subcontractors cannot send workers to the plant when their doses rise. So, he says, subcontractors want to limit exposure on a daily basis.
The man adds that many veteran employees are now unable to do such work because of their high cumulative doses. He says he wants people to understand the current situation involving a less-experienced workforce.