PURPOSE: To allow material in an oscillating tank to be taken out easily at a time as occasion calls.

CONSTITUTION: A safety cover 8 is provided to cover the upper part front of an oscillating tank 1, and an inclined plate 12 inclined downward to the front is located the lower part of the oscillating tank 1 in a frame 6. A front cover 15 with the lower end part inner surface having a water collecting gutter part 17 for receiving water falling from the front end of the inclined plate 12 is arranged at the front face of the frame 6, at the lower part of the safety cover 8. The position close to the inner plate upper end part of the water collecting gutter part 17 of the front cover 15 is pivotally supported by a hinge 21 so as to be rotatable. A series of shoots can be formed by the inclined plate 12 and the front cover 15 by rotating the front cover 15 into such an attitude as to be inclined downward to the front.