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 forcedly supplying water after coagulation to a re- coagulation tank with a pump for subjecting it to coagulation again.

Coagulation treatment of the raw water is carried out in the coagulation tank 1, water after coagulation is force-fed with the pump 3 in the re-coagulation tank 4. 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. The differential pressure of the membrane separation device 5 is measured with the differential pressure measuring device 8. Re-coagulation is properly carried out by controlling the addition of the organic high molecular coagulant by a chemical-feeding amount control device 10 so that the rise value of the differential pressure in a water flow process leading to back washing is within a predetermined allowable range.