An upper end portion of the front leg 13 is connected with a front portion of the armrest member 21 by means of the hinge 23, and an upper end portion of the rear leg 25 is connected with a center with respect to the longitudinal direction of the armrest member 21 by means of a hinge. A center with respect to the vertical direction of the push rod 9 is connected with a rear portion of the armrest member 21 by means of the hinge 27. A lower end portion of the push rod 9 is connected with an upper end portion of the rotary member 29 by means of the hinge 31. A lower end portion of the rotary member 29 is connected with the rear leg 25 by means of the hinge 33. The other end portion of the connecting member 35 connected with the front leg 13 by means of the hinge 37 is connected with a substantial center of the rotary member 29 by means of the hinge 39.