A mis-insertion preventive rib 15 which will fit in the front opening that is formed in the magnetic tape cassette is provided at the storage part 3 which constitutes the magnetic tape cassette storage case 1 and, to facilitate visual and palpable recognition, the height H1 of the lateral wall part 13 which is formed at the outer circumferential part of the storage part 3 is designed so as to be smaller than the height H2 of the mis-insertion preventive rib 15 so as not to be supported between the lateral walls 13 but to protrude from the lateral wall part 13 when the magnetic tape cassette is inserted incorrectly. Therefore, at the time to store the magnetic tape cassette in the storage case, if the mis-insertion preventive rib does not fit in the front opening, the insertion in the incorrect direction can be checked visually by the entire magnetic cassette protruding from the lateral walls. Since the magnetic tape cassette which protrude from the lateral wall part is not supported between the lateral walls, the lid part is difficult to be closed even if tried and thus the incorrect mounting state is quickly recognized.