To sufficiently utilize an opening provided in a seat back, to store a large luggage, and to simply and quickly attaching and detaching an arm rest main body and a back board.

A pivot pin 27 is moved forward and backward by the operation of an operating knob 41 mounted on the arm rest main body 23. The pivot pin 27 is engaged and disengaged with and from a hinge hole 25a of an arm rest bracket 25. The back board 24 is provided on the back side of the arm rest main body 23. The lower part of the back board 24 is rotatably pivoted on a bracket 29. An upper bracket 31 is secured to the upper part of the back board 24. The upper bracket 31 is provided on the upper part of the arm rest main body 23 and engaged and disengaged with and from an upper pivot pin 33. With the arm rest main body 23 drawn, the locking state of the upper pivot pin 33 and the upper bracket 31 is kept and the back board 24 is turned forward.