Two iron pipes planted into the ground and aimed at Camp Zama were found about 800 meters (half a mile) southwest of the base, which sits near a residential area and an elementary school, police said.
No injuries or damage were immediately reported from the incident and police were investigating.
Police found the pipes after a resident reported hearing three explosions in the area in the early hours of today.
Prime Minister Shinzo Abe is on a weeklong tour of the United States, where the two sides agreed late yesterday to a revision of defence guidelines that would boost the Japanese military's role in bilateral cooperation amid China's growing assertiveness in the Asia-Pacific region.