PURPOSE: To effectively extend a retreating area of a footboard without spoiling a return function to a use position, and also, to smoothly execute an initial retreating operation of the footboard.

CONSTITUTION: On the way of turning of a footboard 1, a supporting point is moved to an auxiliary supporting point 24 of the rear side from a main supporting point 22, and a turning operation is e executed in two stages. Also, an initial turning operating operation is executed by a balance operation centering around a balance supporting point 24. In such a way, while always allowing the centroid 0 of the footboard 1 to exists in front of the supporting points 22, 24, the footboard 1 can be allowed to take a large elevation angle. Also, the footboard 1 can be guided smoothly to a reference position Q in which pressing force applied to the front end part 1a of the footboard 1 is converted effectively to the turning moment.