An armrest, has a stationary shaft rotatably supporting an armrest body against a seat frame. The stationary shaft is inserted, so as to have fastening tolerance, inside a coil lock spring and to have a stationary-side hook and a free-side hook. A diameter of the coil lock spring is reduced or enlarged according to a normal or a reverse rotation, respectively, of the armrest body, so as to prevent or permit, respectively, the rotation of the armrest body. One of the inside diameter of the lock spring and the outside diameter of the stationary shaft is tapered in said shaft's axial direction.