It is widely known that shirasu balloons are produced from shirasu. This study, aimed to develop a continuous reactor for producing shirasu microballoons of less than 20 pm in diameter. The effect of the riser-bottom temperature on the bulk density and morphology of shirasu microballoons was investigated. It was found that the bulk density of shirasu microballoons reached a minimum at 1175K and that shirasu microballoons with a particle size of 20 mum or less could be produced.