PURPOSE: To eliminate a special mechanism for preventing misinsertion of a cassette by adding a function for preventing a slider from moving upon misinsertion of the cassette to an operating member for releasing a front lid of the cassette from its locking.

CONSTITUTION: The slider 33 holding the cassette 3 is moved along a guide groove provided on a holder 31 from a standby position to a loading position. A lock releasing arm 41 is provided on one side plate of the slider 33 in a position corresponding to a front lid lock releasing button 5 of the cassette 3. When the slider 33 is started to move from the standby position to the loading position, a base part 41b of the lock releasing arm 41 is shifted from a protrusion 31a. The lock releasing button 5 is pressed by elasticity of a spring with a hook 41c to release the front lid from its locking. When the cassette 3 is not correctly inserted into the slider 33, since the hook 41c cannot be displaced in the direction of the cassette 3, an angular part 43 is restrained by a protrusion 45, and hence the slider 33 is prevented from further moving.