A magnetic tape cassette includes a reel retaining plate for urging hubs of take-up and supply reels vertically downward to achieve uniform tape recording and playback characteristics. The reel retaining plate contains a plurality of protrusions at its central portion which engage with a plurality of vertically extended grooves that are contained in a cylindrical wall which is located on a lower cassette half, the cylindrical wall defining an insertion hole for a light emitting element of a photosensor. The reel retaining plate is held between a top surface of the cylindrical wall and an inner surface of an upper cassette half.