PURPOSE: To necessitate no high pressure capacity with respect to a pump itself and to also remarkably reduce insallation cost, by sending muddy water after temporarily stored under pressure by sending compressed air into a pressure tank.

CONSTITUTION: At first, a water sending pump 12 is rotated to send muddy water sucked up from a raw water tank 11 into a filter press 1 through a water sending pipe 13 and the part of muddy water is flowed into a pressure tank 17 to be temporarily stored therein. A pressure sensor 22 is operated to stop the water sending pump 12 and an automatic opening and closing valve 15 is closed while an automatic opening and closing valve 19 is opened to supply compressed air into the pressure tank 17. The muddy water is directed to a send-in port 4 and pressed by compressed water to receive filtering action. A large amount of a filtrate passed through filter cloth is discharged from the filter press 1 but the discharge amount of the filtrate is reduced and, when a sensor 10 is operated, an automatic opening and closing valve 16 is closed while a fluid cylinder 6 is operated to open filter plates 2 to fall off a cake.