Mechanism for preventing loosening of magnetic recording tape of tape cassette

Within a housing of a tape cassette is mounted a reel hub serving to wind a magnetic recording tape. The reel hub is moved between a first position where one side edge of the tape wound around the reel hub is spaced from a wall of the housing and a second position where the one side edge of the tape abuts against the wall, reel hub being rotated at the first position by a reel shaft of a magnetic recordig tape running apparatus inserted through an opening of the wall of the tape cassette for engagement with the reel hub. Within the housing is also mounted a abutment member. The abutment member is movable between a third position where the abutment member is spaced from the other side edge of the tape wound around the reel hub located in the first position and a fourth position where the abutment member abuts against the other side edge of the tape wound around the reel hub located in the second position by the reel shaft inserted through the opening of the tape cassette housing for engagement with the reel hub. The abutment member is usually urged to the fourth position by a compression coil spring housed in the tape cassette housing.