PURPOSE: To save raw water and decrease waste water by setting an element to control a flow rate of a pure liquid stream, keeping a ratio of the flow rate of the pure liquid stream to that of a raw liquid stream constant and using an impure liquid stream at a pressure lower than that of the raw liquid stream.

CONSTITUTION: An ordinary feed water is obtained by feeding raw water to a membrane module through a pipe 1. A feeding side of raw water of the line 1 is connected to an outing side of ordinary feed water of the line 1 by a bypass line 2. By setting a valve 3 having an over flow function to the bypass line 2 in parallel to the primary side of the membrane module 4, almost constant pressure difference is generated even with a treating liquid amount changing in time. As the pressure difference is constant, constant pure water is produced through a takeout line 7 with a constant rate of raw water stream passing through the membrane module 4.