A rocket hit the Ground Self-Defense Force camp in the Iraqi city of Samawah on Tuesday evening but no one was hurt, the Defense Agency said.
The rocket landed in an empty lot at the base at around 6:50 p.m.
About 600 GSDF troops are currently in the southern Iraqi city to do reconstruction work.
The Japanese troops reportedly sought shelter in freight containers.
It was the ninth attack and the third time a rocket has reached inside the camp. It was the first attack since the Japanese government decided in December to extend the GSDF mission there for one year.
According to Defense Agency officials, the attack came from the west, unlike the other incidents in which rockets came from the north.
The latest attack was also different in that the other rockets were fired during the night or at dawn.
Security in Samawah has deteriorated recently as Iraqis prepare for Jan. 30 parliamentary elections.