Operation force required for pivot operation of a movable body can be set almost constant independent of variation in the angular position of the pivoting. A pivot mechanism (tumble mechanism (10A)) has a floor-side member (B) fixed to a predetermined position; a seat-side member (12) connected to the floor side member (B) by hinge coupling so as to be pivotable in a predetermined angle range; and a torsion spring member (14)provided to urge the seat side member (12)in the opposite direction to gravity that acts on the seat side member (12). The pivot mechanism is constructed such that weight moments about a pivot center (13a) acting on the seat-side member (12) as the pivot mechanism move varies. The torsion spring member (14) has an urging the section (14b) for urging the seat seat member (12) and a support section (14a) that is supported by the floor side member (B). The support section (14b)is disposed at a position between a pivot track of the urging section (14b) produced by the pivoting of the pivot mechanism and the pivot center (13a).