Mounts are provided on the inside surface of the left and the right sides of a seat cover on the underside of a chair, and positioning pins with heads and fastening members are provided on the upper surfaces of the bases of armrests. Each mount includes keyholed-shaped positioning holes respectively having holes for insertion of the heads of the positioning pins and slits of a width suitable for slidably receiving the rods of the positioning pins. Each mount also includes a support hole for the fastening member to be engaged in. When the positioning pins are located at the rear ends of the slits of the positioning holes, the armrest is fixed to the mount through the support hole by the fastening member. Thus the armrests are removably assembled with the chair body. The seat cover has upper and lower protrusions which contact the mount and the base of the armrest, respectively. The mount is bent so that its outboard and inboard portions are inclined relative to each other.