Washing method and apparatus of separation membrane

When a separation membrane is washed with a chemical solution that is a sodium hypochlorite solution, which has a high concentration, diluted with membrane treated water stored in a chemical solution tank, an ammonic nitrogen concentration in membrane-treated water is monitored with a monitoring device, and a control device controls a chemical solution dosing pump using the concentration measured by the device to adjust a dilution rate. Thereby, even when water quality of membrane-treated water varies, a sodium hypochlorite concentration in backwashing water can be retained constant, and fluctuation in the chemical washing effect can be prevented.