PURPOSE: To open a bottom cover with a relatively small power, and a make the discharge of debris and others smoothly without supporting them by an opening part.

CONSTITUTION: A pair of inner cover 3 and outer cover 3 in which a circular arc recessed part is formed on one side are positioned symmetrically on the right and left sides, and pivoted on pins 4, respectively, so that they can be opened and closed. Also an engaging metal fitting 5 provided with an oval hole 4 is fixed to an arm 3a of the outer cover 3 extendedly to the side, and a pin 6 installed projectedly from the inner cover 2 is fitted closely to the oval hole 4. To the arm 3a of the outer cover, a rod 8 lifted by the vertical operation of a handle 13 is connected through a link 11. When the handle 13 is pushed down, first the outer cover 3 is opened, and debris and the others are discharged from the recessed part of the inner cover 2. When the handle 13 is further pushed down and the end of the oval hole is engaged with the pin 6, the inner cover 2 is opened followed by the outer cover 3 so as to expand an opening part. Thus, because the load of debris and the others is applied distributedly on the inner and outer covers, the covers can be opened and closed smoothly.