One person was reported to have suffered minor facial burns from falling debris as Mount Shindake erupted on Friday morning.
Most of the 137 residents of Kuchinoerabu island were evacuated by boat.
One spoke of how the sky turned dark as the volcano erupted.
Flows of rock and super-heated gas travelled down the mountain and into the sea, turning the main harbour grey and the sea-water murky.
Shindake erupted last year for the first time since 1980, when a 14-year period of activity ended. Several people died in a 1933 eruption.
"There was a really loud 'dong' sound of an explosion, and then black smoke rose, darkening the sky," Nobuaki Hayashi, a community leader, told national broadcaster NHK. "It smelled of sulphur.