To surely close a slotlike opening formed through a cover even by an operation of an operation member in a small area.

There are provided a laminated cover member 5 formed by stacking a plurality of cover plates 51 each having different length and a guide member which supports each of the plates 51 movably, back and forth, in a direction, in which an opening 23a of a top cover 23 extends. A slot, through which a connection member 65 of a frame 6 for supporting an pseudo-ski 24 can be passed, is formed in each plate 51. And the length of the slot in the longitudinal direction is made longer as the length of the plate 51 increases. And outer conditions that one of the sides of slot in the longitudinal direction thereof is placed on one of the sides of another slot in the longitudinal direction thereof, the other side of the slot is covered with an end of a short cover plate 51.