PURPOSE: To avoid shock with buffering the open and close operation of cover and to achieve the unit replacement, by providing the mainspring and buffer liquid between two fitting members opposing with each other and rotationable relatively.

CONSTITUTION: When the cassette is put in the cassette loader of the cover 3 being at open position as shown in virtual lines and it is depressed toward the arrow C to close the cover 3, the fitting member 5 and the wheel 13 are turned toward the arrow C to the fitting member 29, latch wheel 20 and wheel 6, and the mainspring 19 is wound to store the energy and the cover 3 is closed. In unloading the cassette, when the stopper fastening the cover 3 is removed, with the energy of the mainspring 19, the fitting member 5 and the wheel 13 are turned toward the arrow C to open the cover 3, and the turning speed is mitigated with the resistive force due to viscosity of the buffer liquid 27 sealed in the gap between the ratchet wheel 20 and the wheel 13 and the wheels 13 and 6.