A fire that erupted Monday night at an Idemitsu Kosan Co. refinery in Tomakomai, southern Hokkaido, was extinguished Tuesday morning, firefighters said.
The fire broke out at around 7:15 p.m. Monday and was put out around 5:30 a.m. Tuesday. Nobody was injured, according to police.
Local police began questioning employees to determine the cause of the blaze and said they would conduct an on-site probe once the area was cool enough to enter.
The fire broke out in a heavy-oil desulfurizer, where hydrogen gas probably leaked from a pipe and caught fire, police said.
According to company officials, 177.4 kiloliters of heavy oil was flowing through the desulfurizer when it caught fire.
Although the flames did not spread to other facilities in the refinery, it took about 10 hours to put out the heavy oil remaining in the desulfurizer that continued to burn, fire officials said.
A fire broke out in a heavy-oil desulfurizer at the oil distributor's refinery in February 2000, and another occurred at the refinery's crude oil refinement facility in December that year.
Police said they would investigate why the desulfurizer has caused another fire, and check whether the firm was properly conducting equipment inspections and maintaining high safety standards at the refinery.
The refinery was established in 1973 and has some 230 employees.