To provide a washing method for a laminated type membrane cartridge from which deposited substances on separation membrane surface can be removed without taking out the laminated type membrane cartridge and provide a filtration system.

A compressed air supplying pipeline 22 branched from a filtrate discharging pipeline 15 of a laminated type membrane cartridge filter 1 connected with an air compressor 20 and compressed air is supplied to the inside of a membrane element from a filtrate outlet 7 to expand the separation membranes. When the separations membrane are expanded, the surfaces of neighboring separation membranes are mutually pressed and sledge cakes deposited on the surfaces are pushed out of the pressured surfaces of the separation membranes. After that, a raw liquid is supplied the pushed out sludge cakes are discharged outside of the layered type membrane cartridge.