Abstract: A system for mechanically and biologically decomposing a garbage discharged from a house, a restaurant or the like at high operational efficiency is disclosed. The system includes a disposer for disintegrating the garbage into a large number of pieces, a solid/liquid separator for separating water from the disintegrated garbage and a garbage decomposing vessel for biologically dissolving the solid disintegrated garbage from which water has been substantially separated in the solid/liquid separator. To promote biological decomposition of the solid disintegrated garbage in the garbage decomposing vessel, pulverized wood is normally used instead of soil as a raw material for forming a decomposing bacillus growing bed in the garbage decomposing vessel. Prior to practical use, the pulverized wood is thermally processed.