PURPOSE: To provide optimum stability in an overhanging state so that a hinge arm is transferred from an inclination position during containing or overhanging to a horizontal position without any trouble.

CONSTITUTION: Gas spring 19 are positioned in respective hinge arms, one end part of the gas spring 19 presses down a hinge arm 12 around a revolving bearing 15 and one end of the other is supported in the region of a cassette 3. This gas spring 19 combines together an advantage wherein elastic stability in an overhang state and an advantage wherein containing movement from the inclination state of a hinge arm 12 to a horizontal state is practicable through elastic bending. This constitution not only eliminates a need for comparatively high-cost mechanical locking and releasing devices for the locking device but also achieves optimum brake operation.