To reduce the number of part item, and to facilitate the fitting work by providing a holder, in which a hinge pin can be freely inserted or removed into or from a U-shaped groove thereof, in both surfaces of a vertical wall of a center hinge bracket, and providing the holder part with a front end surface for sealing an opening thereof, and providing a cam with an energizing means and a stopper means.

Both surfaces of a vertical wall 11 of a center hinge bracket 8 are respectively provided with holder parts 12. In the holder parts 12, a hinge pin 10 is inserted into a U-shaped groove 17 and turnably supported freely. A cam 13 having a front end surface 20 for sealing the opening 16 is provided, and a distance L from a pivotal support point 21 to the opening 16 is formed larger than the maximum diameter D of a head part 9 of the hinge pin 10. A lower end of the font end surface 20 of the cam 13 is provided with a projecting part 22 as a stopper means, which abuts on a lower surface of the holder part 12. A rear end of a torsion bar 14 as an energizing means is passed through a communication hole 23 provided in a rear part of the vertical wall 11, and the cam 13 is energized upward around the pivotal support point 21, and the projecting part 22 is made to abut on the lower surface of the holder part 12 so as to seal the opening 16 with the front end surface 20 thereof.