To produce a raw material for compost or compost itself from garbage which is suitable or raising plants.

There is provided a disposal vessel which is equipped in its interior with a stirring device and a water spray device, and is so constructed that draining is possible from the bottom surface of the vessel only when the pressure is reduced, and into which garbage, microorganisms for use in decomposing garbage, and carriers thereof are charged. Stirring the deposition are repeated for a fixed period of time until these charged substances are converted into compost. When the conversion into compost proceeds to some extent, water is sprayed onto the charged substances (primary fermentation product) while these substances being stirred. Then, the water in the vessel is removed by reducing the pressure inside the vessel with a device for reducing pressure. The saline matter in the primary fermentation product is removed by dissolving into the water discharged from the disposal vessel, so that the primary fermentation product can be used as it is as a raw material for compost or compost itself.