There was an eruption Friday morning off Iwojima Island, some 1,000 km south of Tokyo, but no injuries or damage were reported.
The Meteorological Agency said it was informed by the Defense Agency of the eruption around 10:15 a.m.
U.S. warplanes have been carrying out military exercises on the island since Thursday.
Iwojima last erupted in 1982. There were no major injuries or damage reported at the time.
Some 200 Maritime Self-Defense Force personnel are stationed on the island, along with about 100 Air Self-Defense force members. No civilians live there. The warplanes, which have been conducting takeoff and landing drills on Iwojima, will join the 81,123-ton aircraft carrier Kitty Hawk of the U.S. Navy's 7th Fleet, which left its base at Yokusuka, Kanagawa Prefecture, Friday morning.
The Kitty Hawk is believed to be heading for the Indian Ocean to take part in a possible military response to the Sept. 11 terrorist attacks in the United States.
The meteorological agency said it was told that fumes are emanating from areas close to the shore, which could indicate a sea-bottom eruption, but there have been no reports of magma or volcanic ash being discharged.
The defense and meteorological agencies reported a change in color of the sea surface along the southern coastal area of the island for up to 200 meters out to sea.
The Japan Coast Guard and the weather agency will conduct aerial checks on the eruption to try to determine the exact point and scale of Friday's eruption, the agency said.
Small-scale eruptions have occurred on 10 occasions since the late 19th century on Iwojima.