A magnetic tape cassette having a lower half portion with a photo-sensor inserting hole. The upper half is mated with the lower half to form a cassette body. A tape supplying reel and a tape winding reel are disposed in the body. A reel retaining plate of elastic material is employed for depressing the tops of the reels. The reel retaining plate is fixedly secured to an inner surface of the upper half of the cassette body which is in alignment with said photo-sensor inserting hole.
