A rotational supporting mechanism, which supports a main body and a cover, includes an eccentric cam. The main body and the cover are superposed in a closed position. From the closed position, the cover is rotated 180 degrees and stopped at a 180-degree rotational position in an open position. The eccentric cam rotates eccentrically. The pressing means presses a cam surface of the eccentric cam in a direction of a line connecting a portion of the eccentric cam corresponding to the closed position and a portion of the eccentric cam corresponding to the open position when the main body and the cover are superposed in the closed position. Rotation of the cam is stopped in the open position and the closed position.