A Phillips-type magnetic tape cassette in which widthwise motion of the tape is restricted. The cassette has rotary guide rollers arranged at both ends of the front part of the cassette for guiding the running of the magnetic tape. A shaft supporting each rotary guide roller is cylindrical and has a slit extending in the axial direction and a circumferential annular groove formed in the outer wall in a middle region. Each rotary guide roller has circumferential engaging protrusions extending from the inner wall thereof, which are inserted into the annular groove so that the rotary guide roller is pivotally mounted on the supporting shaft.