PURPOSE: To provide a mechanism for safely expanding and bending a leg sheet part or a step part even when any obstacle exists downward.

CONSTITUTION: The upper ends of an upper oscillation arm 2 and a lower oscillation arm 3 are respectively pivotally attached to the front end of a sitting part sheet supporter 1 by turn shafts 2a and 3a, and the prescribed position of the rear end of a step part 7 is pivotally attached to the lower end of the upper oscillation arm 2 by a turn shaft 2b. Besides, the top end part of an oscillation arm 5b of a leg part driving source 5 is pivotally attached by a rotary shaft 3b at an upper end different from the upper end of the lower oscillation arm 3 or at the prescribed position of its intermediate part. Then, a roller 4 is pivotally attached to the lower end of the lower oscillation arm 3 by a roller rotary shaft 4a in such a manner as to be abutted onto the rear face of the upper oscillation arm 2 at the almost intermediate position and further, a stretch arm outer cylinder 6a mounting a stretch arm sliding bar 6b, whose upper end is pivotally attached to the roller rotary shaft 4a, so as to freely slide it is pivotally attached by a rotary shaft 6c positioned behind the rotary shaft 2b of the step part 7.