Abstract: The magnetic tape cartridge group includes a plurality of magnetic tape cartridges. Regarding 100 reels of magnetic tape cartridges, in a case where a maximum value of an absolute value of a difference between a servo band spacing obtained before predetermined storage and a servo band spacing obtained after the predetermined storage is defined as A, an arithmetic mean of medium lives calculated by a linear function derived from a value of A obtained for different N and a value of the logarithm of total storage time T of the storage of N cycles is 3 years or longer and 3? is 1 year or shorter. Medium life: T in a case where A satisfies A=1.5?B. B: a value calculated by multiplying a difference between a maximum value and a minimum value of servo band spacings obtained in each of the five environments by ½.

Abstract: A sample is placed on a sample receiving stage for receiving the sample. The sample is transferred one at a time to a measuring table, and a thickness of a thin film formed on a surface of the sample is measured by irradiating the surface of the sample with a measuring light beam. A covering structure is disposed between the sample receiving stage and the measuring table to define a sample transfer space and a measuring space in which the measuring table is disposed. A high-purity purging gas containing only a very small amount of contaminants is supplied through purging gas supply devices into the transfer space and the measuring space covered with the covering structure.