OTAKI, Chiba -- A police officer here got a little more flame than he was expecting when he flicked on his lighter in a police box men's room early Friday and an explosion opened a seven square meter hole in the ceiling.

The 51-year-old officer was sent to hospital with minor burns on his face and hands.

According to police, there were no explosive materials in the washroom, and when another officer came running after hearing the blast he found a 1.5 meter pillar of flame spewing from the drain in the floor. It's thought the washroom was filled with gas from a natural gas pocket that had leaked into the drain pipe. Investigators are now searching for the source of the gas.

Otaki is in the center of the Boso Peninsula, the location of the first natural gas well in Japan, drilled in 1891. According to the prefecture, the peninsula is in the center of the southern Kanto gas fields -- the broadest in the country by area and producing the second highest volume of gas, at about 463.22 million cubic meters per year (as of 2008). Kanto Natural Gas Development Co., which developed the fields, says that the gas is 99 percent methane, which is odorless.

The region has seen a number of explosions believed to have been caused by natural gas since 1988, including the July 2004 explosion of a museum in the town of Kujukuri in which an employee died and one other person was injured.