The present invention purports to provide a tape recording/reproducing apparatus capable of causing a plurality of tapes having different width respectively to travel smoothly and its cassette. The cassette positioning spring 1 gives an appropriate pressing force even in the case that the cassettes have different thicknesses, and the cassettes are respectively fixed at predetermined positions so that the lower edges of the magnetic tape 3a, 3b are positioned at the same height. The flange 9 of the guide post 12 is shifted in an up-and-down direction in response to a magnetic tape width signal detected through the cassette detecting switch 13 or the beginning edge detector 14a, or the ending edge detector 14b, so that even narrow magnetic tape 3a can travel at the predetermined height. Furthermore, by changing the inclined angle and/or the inclined direction of the movable post 10, a downward torsion force is applied to the narrow magnetic tape 3a. In this fashion, magnetic heads to be used are selected in accordance with the magnetic tape width detected by the cassette detecting switch 13 or the beginning edge and the ending edge detectors 14a, 14b.