PURPOSE: To prevent the increase in filtering resistance with increasing thickness of filter cake layer by advancing filter cloth while changing it into a wave-like shape by supporting the bottom filter cloth of a ship bottom-shaped concavity on plural supporting rolls.

CONSTITUTION: During a process in which blockish solids gradually deposit on the filter cloth 1 to form the filter cake layer D, the bottom filter cloth 1 of the ship bottom-shaped concavity A supported by the supporting roll 5 group is advanced while being changed its shape, and then the filter cake layer D is cracked E above the supporting roll 5 and compressed before and behind at its lower part, whereby it is dewatered up to a state of moist filter cake layer without the increase in filtering resistance with increasing thickness of the filter cake layer D. Afterwards, the filter cake layer is rapidly dewatered during the passage through the suction-dewartering box 7, and then the dewatered filter cake is scraped by the scraper 12 on the main roll 3. Then, the filter cloth 1 is passed through the filter cloth cleaning box 8 where it is cleaned by the actions of the air permeating through the filter cloth and the air stream abrading the surface of filter cloth which result from vacuum suction.