Mount Shindake erupted at about 10 a.m. shooting towering black-gray plumes of smoke and ash into the sky.
There were no injuries reported, although the Japan Meteorological Agency reported that pyroclastic flows from the volcano had reached the shore to the northwest.
Japan's top government spokesman, Chief Cabinet Secretary Toshihide Suga said there was no threat to human life after the eruption.
About 140 people live on Kuchinoerabu island in the Ryukyu archipelago, which is approximately 50 miles (80 kilometers) southwest of the main southern island of Kyushu.
Japan's national broadcaster NHK reported local officials as saying that 80 residents had taken refuge in a shelter on the island.
Kuchinoerabu island is only accessible by ferry. It runs once a day from Yakushima island which has an airport and a population of more than 13,000 people.
Shindake also erupted in August last year for the first time since 1980.
av/bw (AP, AFP, Reuters)