Japanese media are reporting today the government is investigating a possible battery defect in the iPod Nano, sold by Apple Japan.

Apparently, one iPod Nano (model MA005) scorched a Japanese traditional “tatami” mat (made of woven soft rush straw) in January. Another model, the MA099, singed nearby paper in August.  Both incidents took place in Tokyo but even though the devices were destroyed, the fires didn’t cause any further damages or injuries.

The Ministry of Economy, Trade and Industry is currently in contact with Apple Japan to find out the reasons for the defects, suspecting problems with the Nano’s lithium-ion battery. According to various reports, the iPods began to overheat while they were being charged.

Apple was instructed to report back to the Japanese government as soon as possible. A similar case was investigated in Japan in March, when an iPod Nano began shooting sparks.

iPods are Japan’s most popular portable music players and these incidents might lead to trouble for Apple Japan, given Japanese customers always expect to get absolutely flawless products.

It’s unclear whether the batteries, which caused the accidents in Tokyo, are used in iPod Nanos outside Japan, too.