This invention aims to provide a portable terminal device capable of easily and accurately measuring and evaluating mental fatigue, its measuring method, and a server computer. The portable terminal device includes an operation unit 20 an imaging unit 18 for measuring ambient light a display screen 16 or light emitting element 17 for displaying a blinking image while a frequency of the blinking image is being changed and a \ recording unit 14 for recording a flicker frequency, as a measurement frequency, at the time when a user operates the operation unit 20 to indicate that the user perceives flicker during the display of the blinking image. 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.
