PURPOSE: To discriminate the digital compact cassette(DCC) and analog compact cassette(ACC) of the magnetic recording and reproducing device by detecting the opening of the shutter provided on the DCC.

CONSTITUTION: A locking member 4 is freely turnably mounted to a sliding member 2 freely slidably provided on a back guide 1. The locking member 4 is energized to the shutter 102 side of a cassette pack 100 by a spring 44. The shutter opening member 43 at the front end of the locking member 4 is moved into a recessed part 104 by the force of the spring 44 and is engaged with the closing side end of the shutter 102. The shutter 102 is detained by the opening member 43 and only the cassette pack moves inward when the cassette is pushed in by hand. An actuating member 46 presses a pressing piece 48 by the turning position of the locking member 4 when the opening member 3 intrudes into a recessed part 104. A cassette discriminating switch 47 is thereby actuated and the DCC is discriminated. The locking member 4 does not turn in the case of the ACC without having the recessed part and, therefore, the cassette discriminating switch 47 is not pressed.