PURPOSE: To wash a filter membrane while preventing the deterioration of the filter membrane by circulating a city water until a water temp. reaches a prescribed temp., injecting a washing liquid chemicals to a circulating city water when the filter membrane of a membrane separator is washed by circulating a heated washing water.

CONSTITUTION: The filter membrane is washed by the circulating system 11 circulating a washing water to the membrane separator 1 having the filter membrane 2 by a circulating pump 12, the heater 14 heating the water flowing through the circulating system and the thermometer 20 measuring the water temp., a chemical injecting pump 22 injecting the washing liquid chemicals to the circulating system and the controller 24 driving the chemical injecting pump when the thermometer detects the prescribed temp.