PURPOSE: To save energy by reducing power consumption as a whole by forming tube filter into a turning-back shape or a meandering one.

CONSTITUTION: Both ends of many porous tubes of a filter 4 formed each into a U-shape by turning it back are joined to the supporting plate 2 of the filter 4 by welding. And when a feed liquid is forced to flow into an inlet chamber 3a by a circulating pump, a part of the liquid permeates the tube walls and flows out as a filtrate to the inside of a tower 1 while the liquid flows in the inside of each tube elements 4', 4' connected to each other by a stainless bend 6, and a concentrated untreated liquid flows into an outlet chamber 3b. Therefore the treated water is taken out from the inside of tower 1 through a take-out hole 7, and the concentrated untreated liquid is returned to a circulating tank and is mixed with the untreated to circulate by forcing the mixed liquid to enter the chamber 3a by the circulating pump. As a result, the flow rate of liquid is reduced without raising the delivery pressure of the pump so much, and the required energy is saved by reducing the power consumption as a whole.