To provide hinges to be mounted on thin materials without being exposed to usage planes and to be folded with the usage planes directed inside.

As hinges 1 are mounted on the backs of beds 10, 11 not to be exposed to usage planes, the whole usage planes of the beds are comfortably used. As mounting plates 2, 3 are mounted on the backs instead of the sides of the beds 10, 11, the hinges 1 can be mounted even on the thin beds. In addition, the hinges 1 are mounted at the centers on both sides of two beds 10, 11 and the height of side plates 4, 5 is equal to the thickness of the beds 10, 11, so that both beds 10, 11 can be completely folded with the usage planes of the beds 10, 11 directed inside, resulting in the usage planes not to be contaminated during storing the beds.