PURPOSE: To improve the operability of a cover just before completing the closing operation of the cover, by constituting a driving gear and a transmission gear of plural gear groups engaged and arranged with each other selectively, respectively, and switching and engaging them with gears whose speed ratios are decreased just before completing the closing operation of the cover.

CONSTITUTION: The transmission gear 22 is provided with the gear 23 small in diameter and the gear 24 large in diameter formed integrally from the side of a device main body 1 centering a supporting shaft 21. By applying a pressing operation in a direction to close the cover 3, the resiliency of an elastic member 10, etc., which always presses the cover 3 in a direction of the opening operation of the cover 3 so as to restore a cassette holder from a mounting position to a dismounting position just before completing the closing operation of the cover 3 arrives at peak, however, the gears are switched to and engaged with the gears whose speed ratios are decreased automatically out of the gear groups of the driving gear 5 and the transmission gear 22 provided at the base part of the cover 3. In such a way, an operating force at the time of closing the cover 3 manually can be reduced, thereby, the operability can be improved remarkably.