Abstract: A tape cassette having a swingable cover adapted to cover and protect a tape stretched along the front side of the tape cassette, a cover locking member adapted to prevent swinging of the swingable cover by rotating in the thicknesswise direction of the tape cassette within a groove which is defined between the inner surface of one side wall of the swingable cover and the side wall of the box-like structure of the tape cassette and also between two side walls of the box-like structure which are on the extensions of the one side wall of the swingable cover and the above-mentioned side wall of the box-like structure. The groove serves also as a mis-insertion preventing groove for preventing the tape cassette from being inserted into the cassette holder in a wrong posture. According to this arrangement, it is possible to realize a compact construction while fulfilling all of the requisites for the tape cassettes.