1. A portable terminal device capable of measuring mental fatigue comprising: an operation unit; an imaging unit for measuring ambient light; a display screen for displaying a blinking image while a flicker frequency of the blinking image is being monotonically changed with time from a start frequency to an ending frequency; and a recording unit for recording the flicker frequency, as a measurement frequency, at the time when a user operates the operation unit to indicate that the user perceives flicker during the display of the blinking image, wherein: first frequency data, which is the measurement frequency measured when the user is specified as being healthy via the operation unit, is associated with first environmental data, which represents the ambient light measured by the imaging unit, and the associated data is stored in the recording unit; and a proportion of decrease of second frequency data, which is the measurement frequency measured when the user is not specified as being healthy via the operation unit, from the first frequency data associated with the first environmental data having the same order of magnitude as that of second environmental data, which represents the ambient light measured by the imaging unit, is calculated to evaluate a degree of fatigue of the user, and the degree of fatigue is displayed on the display screen, and wherein: each of the first and second environmental data is a value calculated by 
 K×S n /S s , using image luminance data S s obtained by capturing a reference image at a position having an illuminance of K; and image luminance data S n obtained by the imaging unit capturing the reference image at a position having an arbitrary illuminance.