Volcanic ash rained down on the west flank of Mount Soputan in Minahasa district, accompanied by mild tremors, said Ahmad Jumarna, director of volcanology in West Java.
"There's a dome containing 23.5 million cubic meters of lava," Jumarna told Elshinta radio, adding that the volcano had erupted many times since the 16th century.
The volcano lies 23 kilometers (14 miles) from residential areas, Jumarna said.
Soputan erupted in July 2000, shooting lava 50 meters into the air. Explosions and ash emissions continued until August 2000.
The Indonesian archipelago has some 127 active volcanos, along what is known as the Pacific Ring of Fire.