The present invention provides a method for washing a separation membrane module after filtering raw water containing particles having a hardness higher than that of a separation membrane. Thus, after completion of filtration, water at the primary side in a separation membrane module is drained to outside the system, backwash waste water in the separation membrane module is then drained while carrying out backwash, and any of the steps of: (a) filling the primary side in the separation membrane module with water and carrying out air scrubbing; and (b) carrying out air scrubbing while feeding water to the primary side in the separation membrane module is then carried out, followed by draining water at the primary side in the separation membrane module to outside the system.