A tape cassette of this invention is equipped with a memory capable of accommodating management information. The memory retains an MIC logical format type designating a format state, either formatted or unformatted, of the magnetic tape. A check is made on the consistency between the format state designated by the MIC logical format type and the format state estimated by reading data from the magnetic tape. If an inconsistency is detected, that is interpreted to mean that the tape cassette has been tampered with, and read and write operations on the cassette are controlled accordingly. This enhances security against fraudulent uses of a tape drive system loaded with the tape cassette.