To continuously subject a large quantity of water to be treated containing phytoplankton to filtering treatment at a stable speed.

A drum 12 is arranged in a water tank 18 and two pairs of feed rolls 14a, 14b, 16a, 16a are arranged so as to hold the outer peripheral surface of the drum 12 therebetween and delivery roll, 20 is arranged in connection with the feed rolls 14a, 14b and, further, a taking-up roll is arranged in connection with the feed rolls 16a, 16b. Filter cloth 24 for removing phytoplankton reaches the taking-up roll 22 from the delivery roll 20 through the respective feed rolls and the drum 12. Since the filter cloth 24 successively moved by the rotation of the respective rolls and the drum 12, a new surface always comes into contact with water to be treated in the water tank 18.