A ratchet 3 is secured to a rotatable base plate structure 2 of an arm rest device 20. A Y-shaped pawl member 7 is pivotally attached to a stationary plate 6 by a pivot shaft 14 and a toggle spring 8. The stationary plate 6 includes a proximal end portion fitted over a fixation shaft 5. A tooth pawl 13 formed in one end portion of the Y-shaped pawl member 7 is engaged with a row of serrated teeth 10 of the ratchet 3 within a predetermined extent (from point A to point B) of pivotal movement of the base plate structure 2, by reason of urging load P from the toggle spring 8. Thus, the arm rest body 18 is fixed at a predetermined position. An engagement groove 32A in the Y-shaped lever 32 prevents the arm rest body in an accommodated position from being pivoted in a forward direction.