Abstract: A magnetic tape cassette in which loop formation during slackening of the tape is prevented and the tape is wound on the winding hub without being displaced laterally, thereby preventing jamming of the cassette and deformation of the tape. The cassette includes a pair of hubs on which the magnetic tape is wound, two guide rollers, one at either end of an opening formed in the front of the cassette, and two inner guides, one provided between each of the guide rollers and a respective one of the hubs. Each inner guide includes a cylindrical barrel having a tape sliding surface, and a protrusion extending from the cylindrical barrel. The protrusion, which has a tape sliding surface wider than the tape, is turnable rearwardly of the cassette. A tape guide groove is formed in the tape sliding surface of the cylindrical barrel having a width slightly greater than that of the tape.