To provide a method for washing a membrane separation apparatus which can eminently reduce the amount of a chemical liquid to be used to improve a separation membrane washing effect and to obtain a prescribed washing effect by washing separation membranes with the chemical liquid after piping to the membrane separation apparatus is washed efficiently and a device for the method.

In the device for washing the membrane separation apparatus, a washing water discharge pipe which does not practically introduce washing water supplied to a raw water introduction pipe into the membrane separation apparatus, makes the washing water pass through the raw water introduction pipe, and discharges the washing water is connected to raw water introduction pipe for introducing the raw water into the membrane separation apparatus. After the piping is washed, the separation membranes are washed with the chemical liquid.