an acceleration / deceleration mechanism is installed for a lid body rotatably supported on a main body . the mechanism includes a forcing device for forcing the lid body toward an open direction , a locking device for maintaining a closed state of the lid body relative to the main body in opposition to the force of the forcing device , and an acceleration / deceleration device situated between the lid body and the main body . the acceleration / deceleration device increasing a speed of opening of the lid body at less than a prescribed angle of opening of the lid body , and decreasing the speed of opening in excess of the prescribed angle of opening .