A process for dehydrating water-containing organic solid material which comprises steps of introducing the material into a pressure vessel, applying saturated steam to said vessel to heat the material so that water in the material be removed and mixed with condensate of the steam to be exhausted from the vessel as a waste water. The process is characterized by a step of charging material of relatively large particle size at a bottom portion of the vessel and of relatively small particle size on the material of large particle size. The process makes it possible to dehydrate relatively fine particles of material by utilizing liquid form dehydration phenomemon.