Coralloid speleothems are commonly distributed in Togawa-Sakaidani-do Cave in Miyazaki Prefecture, Central Kyushu, but their speleological study has not heretofore been achieved. Light and scanning microscopes analyses revealed that coralloid speleothems consist of alternating layers of diatom colonies, detrital minerals and clay. Electron microprobe analysis shows coralloid speleothems to be silicious. This paper asserts that diatom (genus Melosira) is one of the important contributors to siliceous coralloid speleothems in the threshold zone at non-calcareous caves