The bomb that exploded near Miyazaki International Airport was an improvised explosive device from World War II, Reuters reported.
The explosion created a hole near the runway, one meter deep and seven meters wide.
A bomb disposal team from the Japanese ground forces investigated the incident and found that the cause of the explosion was a US-made bomb that was probably used in an airstrike during World War II in the area, a Transport Ministry official said. It was buried underground.
The incident occurred at 9:00 am local time, canceling 87 flights at the airport, but no one was injured, the report said.
Miyazaki is the capital of the province of the same name on the island of Kyushu in southwestern Japan.