PURPOSE: To perform control of a dust catcher and a regulating door body by means of a signal for a rotation speed difference by a method wherein water levels on the upper stream side and on the downstream side are detected as the rotation speed difference of a rotary shaft.

CONSTITUTION: When dust is adhered to a dust catcher, for example, a bar screen 5, a water level on the upper stream side where a float 22 is provided is raised to a value higher than a water level on the downstream side where a float 32 is provided, the float 22 is raised to a level higher than that of the float 23 according to a water level difference. Simultaneously, a first pulley 25 is rotated. In this case, since, though a first bevel gear 42 is caused to be rotatable, a second bevel gear 43 is not caused to be rotatable, a rotation speed difference is capable of being produced between the two bevel gears 42 and 43. By means of a so formed differential gear mechanism 40, a cam 41 is slowly rotated and a limit switch 50 is turned ON. By means of an input signal to the limit switch 50, a rake 6 is raised along a screen 5 to rake up refuse.