PURPOSE: To treat raw water containing a large amount of hard components easily at low cost, keeping the deterioration of a composite membrane at low level by treating the raw water in the presence of an oxidizing agent by means of a synthetic polymer composite membrane module after adding a chelating agent to the raw water.

CONSTITUTION: While a chelating agent such as ethylenediaminetetraacetic acid and an oxidizing agent such as sodium hypochlorite are being injected by means of injection devices 3, 4 for chelating agent and oxidizing agent respectively, a liquid feed pump 5 is driven to feed raw water to a synthetic polymer composite membrane module 2. And the raw water is filtrated by a cross flow method and the water passed through the module 2 is taken out from a water takeout pipe 23, while a part of raw water which is not passed through the module is being returned to a raw water tank 1 through a line 81. During processing the raw water, the oxidizing agent injection device 4 is controlled by means of an oxidizing agent sensor 9 to adjust the concentration of chlorine on the raw water side 21 of the module 2 to within the range of 0.1-1.0ppm.