A magnetic tape device. The angle θ formed by the axis of the element array of the magnetic head with respect to a width direction of the magnetic tape is changed during running of the magnetic tape in the magnetic tape device. In a case where predetermined storage is defined as one cycle, a maximum value of an absolute value of a difference between a servo band spacing obtained before performing the storage and a servo band spacing obtained after the storage of N cycles is defined as A, and N is set to 1, 2, 3, 4, or 5, a medium life calculated by a linear function of A and a logarithm loge T of T, that are derived from a value of A and a value of the logarithm loge T of total storage time T of the storage of N cycles is 5 years or longer.