CNN said a subcontractor working on the site in December told its personnel to place lead shields over their personal radiation monitors in order to reduce the recorded levels of radiation.
"We will firmly deal with the matter once the practice is confirmed to constitute a violation of any law," said an anonymous official at the Ministry of Health, Labor and Welfare.
The operator of the Fukushima Daiichi nuclear plant denied any knowledge of the shields, which the Japanese newspaper Asai Shimbun said Saturday were designed to fit over the dosimeters workers carried in the pockets of their protective suits.
Fukushima was seriously damaged by an earthquake and tsunami that struck Japan nine months before the reported incident.
Notable deaths of 2014 [PHOTOS]
Ebola health workers killed in Guinea