PURPOSE: To develop the capacity of a membrane separation apparatus stably over a long time, by treating raw water by a micro-strainer or by the micro- strainer and subsequently by a sand filter while a coagulant is added to raw water to form flocks.

CONSTITUTION: A coagulant is added to raw water taken in by a water intake pump 1 from a coagulant tank 5 through a coagulant injection pump 2 and sent to a micro-strainer 6 while flocks are formed. In this micro-strainer, suspended substances are removed and treated water is sent to a sand filter 3 through a pump 7 to further remove residual turbid substances. The filtrate is further sent to a cartridge filter 4 and turbid substances are perfectly removed by utilizing respective characteristics of each aforementioned filter means.