To reduce the frequency of chemical cleaning of a membrane solid- liquid separation device by stabilizing the flux of a membrane after adding coagulants in raw water such as river water, ground water, lake water, industrial water or various wastewater and carrying out a coagulation reaction in a coagulation tank and forced supplying water after coagulation to a re- coagulation tank with a pump for subjecting it to coagulation again.

An inorganic coagulant, an organic high molecular coagulant, and a concentrate from the membrane separation device 5 are added to the raw water, and coagulation treatment is carried out in the coagulation tank 1. The water after coagulation is supplied forcedly to the re-coagulation tank 4 with the pump 3. The organic high molecular coagulant is added and flocks crushed by the pump 3 are re-coagulated, then the solid-liquid separation is carried out by the membrane separation device 5.