A portable terminal device capable of measuring and evaluating mental fatigue, its measuring method, and a server computer is provided. The portable terminal device includes an operation unit an imaging unit a display screen or light emitting element and a recording unit. Environmental data, which represents ambient environmental light, is calculated by capturing at least a reference image, first frequency data measured when the user is healthy is associated with first environmental data at the time of the measurement, and the associated data is stored in the recording unit and a proportion of decrease of second frequency data measured when the user is not healthy from the first frequency data associated with the first environmental data having the same order of magnitude as that of second environmental data at the time of the measurement is calculated to evaluate the degree of fatigue of the user.
