To lessen the deviation of dehydration and to improve dehydration efficiency by building spherical seats into pressure plates at both ends and predehydrating a slurry under specified pressure in the filter cloth arranged between these pressure plates by pressurization injection prior to actuating the pressure plates, then actuating the pressure plates.

A hydraulic pump is started and the pressure plates 14a, 14b are opened before the start of operation. The filter cloth 21 is hung at the hooking metals of the plate surfaces and quick cups 30a, 30b of a connection cylinder port 29 are coupled. At this time, the connection cylinder port 29 is held 'open'. The slurry is supplied into the filter cloth 21 by putting a selector valve 40 to 'open' and a closing means 22 of the filter cloth 21 to 'close' and starting a slurry supply pump. The slurry is predehydrated by the injection pressure of the slurry supply pump at this time. When the inside of the filter cloth 21 is full, the selector valve 40 is 'closed' and the hydraulic cylinder is set at 'advance' to start the compression by the pressure plates 14a, 14b. The hydraulic cylinder and the pressure plates 14a at both ends are connected via the spherical seats at this time and, therefore, the slurry injected into the filter cloth 21 is evenly compressed.