PURPOSE: To reduce the load on a dust collector even at generation and inflow of a large quantity of dust and maintain the water level difference between the upper stream and the lower stream of the dust collector within a certain limited range by providing a preliminary dust collector made of perforated flat plate upstream from the dust collector.

CONSTITUTION: A dust collector 100 in which a screen frame 4 provided with a screen for collecting dust is joined with an endless link chain 3 and the link chain 3 is driven by means of a pair of upper and lower sprockets 1, 2, is provided. Next, a preliminary dust collector 200 composed of a perforated flat plate 240 crossing a water passage 12 and a pair of right and left perforated flat plates 250, 250 one-side ends of which are connected to the inner wall of the forward water passage and the other-side, ends are connected to the middle part of the perforated flat plate 240, is provided upstream from the water passage of the dust collector 100. When a large quantity of dust such as jellyfish is generated and let flow in, large dust is caught by the perforated flat plates 240, 250 of the preliminary dust collector 200, the load on the dust collector 100 is reduced, and water level difference between the upper stream and the lower stream of the dust collector is maintained within a certain limit.