PURPOSE:To enable dust on a rake to be dropped on a dust plate exactly by a method in which a toothed plate at the tip of the rake is made to do a raking- up work along a comb-shaped screen and a scraper in the raking-up position of the rake is moved in a rocking manner. CONSTITUTION:When a rotary shaft 11 to be pushed by a rocking cylinder 12 is turned, a rake 3 is made to do a downward rocking and creeping action into water channel 1. At the lower limit of rocking of the rake 3, a retracting cylinder 22 does a pulling action and the rake 3 moves in an upward rocking manner around a fulcrum shaft 20. A toothed plate 26 at the tip end of the rake 3 is engaged with a comb-shaped screen 2, the rocking cylinder 12 does a pulling action, and by the inverse rotation of the rotary shaft 11, the rake 3 moves in an upward rocking manner in water channel 1. Dust (a) collected by the comb- shaped screen 2 is raked up by the toothed plate 26 of the rake 3, and then a scraper 4 does a pulling and rocking action around the fulcrum shaft 29 to drop and deposit dust (a) on the rake 3 onto a dust plate 33.