PURPOSE: To obtain a cassette alowed to be inserted to a VTR used both for standard recording and high density recording but interrupted from being inserted to a VTR used for standard recording with a different recording format.

CONSTITUTION: To prevent the erroneous insertion to a magnetic recording and/or reproducing device, an erroneous insertion preventing groove part 2a62 coped with the time of insertion from the wider width direction and erroneous insertion preventing groove parts 2a82, 2a84 coped with the time of insertion from the narrower width direction are respectively provided at prescribed positions on the bottom surface 2a of a 2nd cassette casing 2B constituting the tape cassette 1B for high density recording. These erroneous insertion preventing groove parts 2a62, 2a82, 2a84 are formed on the bottom surface 2a of the 2nd cassette casing 2B constituting the tape cassette 1A for standard recording and made to prescribed dimensions which are short by the extent capable of visibly confirming from an erroneous insertion preventing groove part 2a61 coped with the time of insertion from the wider width direction and an erroneous insertion preventing groove parts 2a81 coped with the time of insertion from the narrower width direction.