To realize a structure of low equipment cost and continuous supply of membrane permeate.

While monitoring the liquid level of a slurry tank (1) by a level meter (2), a filtration process is carried out by one of a plurality of filter membrane units (7a, 7b). When the liquid level of the slurry tank (1) decreases to a backwashing start level B, a backwashing process is carried out by the other of a plurality of the filter membrane units (7a, 7b) to supply backwashing waste liquid to the slurry tank (1). When the liquid level of the slurry tank (1) increases to a backwashing stop level T, the filtration process by the one of a plurality of the filter membrane units (7a, 7b) is stopped, and the backwashing process by the other is switched to a filtration process. Thereby, both controllers for a backwashing flow rate and backwashing time become unnecessary, realizing a structure of low equipment cost. The filtration process is carried out by at least one of a plurality of the filter membrane units, which enables continuous supply of membrane permeate.