The Meteorological Agency said Saturday that Mount Oyama on Miyake Island erupted at around 6:43 p.m. after confirming that white smoke was billowing out of the volcano.
The agency said the smoke from the mountain had reached heights of some 800 meters above the peak and that it was flowing in a northeasterly direction.
On June 29, the mayor of the village of Miyake decided to lift an evacuation order for all residents after volcanologists declared the island safe just days after seismologists had issued a warning that the mountain might erupt.
Experts confirmed at the time that there appeared to have been an undersea eruption off the western coast of Miyake Island.
Officials of the Coordinating Committee for the Prediction of Volcanic Eruptions, an advisory panel to the Meteorological Agency, told a news conference at that time that there was almost no possibility of the 813-meter mountain erupting.
Miyake Island is part of the Izu island chain, which has been experiencing thousands of earthquakes over the past few weeks. The largest of these hit nearby Kozu Island on July 1, killing one man in a landslide.
Mount Oyama last erupted in 1983, destroying 400 houses. It has a population of roughly 3,800.