There were no immediate reports of injuries or damage following the eruption of the 2,568-meter (8.474-foot) Mount Asama, 150 kilometersmiles) northwest of the capital.
"There is a need for caution but it is not st the level where people need to run away," said agency official Yuji Nishimae.
Rocks from the volcano were thrown over a wide area, the agency said, without elaborating.
Television footage showed burning red embers at the volcano's cap. Nishimae said it was not clear if that meant lava had escaped.