PURPOSE: To easily execute parting of dehydrated cake by making rough surface of a piston at the time of dehydrating slurry raw material by pressing the piston and also inserting flat sheet having elasticity between the rough surface and the slurry raw material.

CONSTITUTION: The slurry raw material is charged in a cylinder 2 and the piston 3 is pressed down by hydraulic press, etc., to compress the slurry raw material. Water is drained from holes of a filtrating plate by this compression and the dehydrated cake 11 is remained in the inner part of the cylinder 2. Then, as urethane sheet 9 is stuck to surface of the piston by compressing, surface of the dehydrated cake 11 becomes rough surface corresponding to the piston surface. After producing the dehydrated cake 11, the pressure loaded to the piston 3 is released, to return to atmospheric pressure, and then, the urethane sheet 9 returns to the original flate-state from the rough surface by elastic force, which the urethane sheet 9 itself has, and the urethane sheet 9 and the dehydrated cake 11 are naturally separated.