To enable miniaturization of a platform.

A flapper 62, which is provided turnably at a back edge 61 of the platform 33 which is made to ascend/descend by a lifter, is split into a center flap 63 in a central part and side flaps 64 in both side parts, and made independently turnable. Both the side flaps 64 are coupled together by a coupling bar so as to be interlocked, and supported by a supporting mechanism 71. The supporting mechanism 71 keeps both the side flaps 64 in a closed state of being raised with respect to the platform 33 and an opened state of being inclined toward a back lower part. The center flap 63 is supported on the back edge 61 by a supporting shaft 91, and an end of the center flap 63 is urged in a backward turning direction by a coil spring 92 of the supporting shaft 91. An end of a supporting bracket 96 is extended out to a lower part of the center flap 63 from an under surface of a second plate 42, and the urged center flap 63 is kept in a downward inclined state.