To provide a method for washing a separation membrane, capable of attaining a stable filtration performance for a long time by effectively dissolving substances adhering to or blocking-up the filtration pores of the separation membrane.

In a method for washing a separation membrane which performs solid/liquid separation by being immersed in a treatment liquid in a facility for the biological treatment of organic foul water such as sewage, hot water F is injected into the separation membrane 1 from the permeate effluent side, and a hydrolase solution G is poured into the separation membrane 1 from the permeate effluent side.