Abstract: A tape cassette has a case accommodating magnetic tape wound around a pair of reel hubs. The case defines tape feed path openings extending through both ends of the case near its front portion from winding openings to a front recess through which a magnetic head or the like is to be inserted. The case has a base portion and top and bottom plates covering top and bottom surfaces of the base portion. The base portion is solid in the direction of its thickness except at the winding opening and the tape feed path openings. The base portion has a front portion and a rear portion contiguous to each other through a bridge extending between the winding openings. The base portion and the top and bottom plates are cut out at portions corresponding to the front recess. The base portion is provided at the front part thereof with guides to guide the magnetic tape. An adaptor may be provided for use with the tape cassette. The adaptor has a case into which the tape cassette is insertable.