Police searched Idemitsu Kosan Co.'s oil refinery in Tomakomai, Hokkaido, on Saturday on suspicion a recent series of huge flammable liquid fires sparked by earthquakes was a result of negligence.
Investigators want to check the company's maintenance and inspection methods to determine whether Idemitsu broke the law in reacting to the blazes last month.
Two major earthquakes that hit southern Hokkaido on Sept. 26 damaged oil storage facilities at the refinery and caused a fire in a crude oil tank. A naphtha tank ignited Sept. 28, causing an enormous conflagration that took 44 hours to extinguish.
The tanks had been damaged in the earthquakes, but Idemitsu allegedly failed to inform fire authorities, as required by law.
The company also did not have a manual on steps to take in the event of an earthquake.
There have been six fires, including the two after the quakes, at the refinery since February 2000, and the latest blazes drew hundreds of complaints from local residents, although no injuries were caused.