PURPOSE: To easily and uniformly wash a membrane module without damaging a membrane or obstructing the flow of water to be treated.

CONSTITUTION: A membrane separator 32 is immersed in a treatment tank and treated water is separated from water to be treated by the membrane separator 32. The water to be treated in the treatment tank is discharged so as to leave a set amt. of water to be treated and the membrane module 41 of the membrane separator 32 is exposed to a gas phase. Next, the remaining water to be treated is taken out of the bottom part of the treatment tank to be supplied to a sprayer 35 and ejected to the membrane module 41 from the sprayer 35. Since the surface of a membrane is exposed to the gas phase, the momentum possessed by water to be treated can be allowed to directly act on the surface of the membrane. Therefore, the solid stuck to the surface of the membrane can be sufficiently peeled.