PURPOSE: To discriminate the winding diameter of a tape cassette for a video cassette, etc., even during the tape running is stopped and also to discriminate the accurate winding diameter at the time when the tape is running.

CONSTITUTION: Hole parts 1c, 1d or transparent material parts (not shown in figure) are provided at the bottom side 1b of a case 1a for the tape 'cassette 1, and reflection type photosensors 2c1, 2c2, 2c3, 2d1, 2d2, 2d3 are provided opposing thereto in a tape cassette driving device 2, then the winding diameter is discriminated by the operation of the photosensors. The winding diameter can be discriminated even during the tape is stopped, and the winding diameter is accurately discriminated during the tape is running since there is no air caught into the tape, and the like.