A method for measuring mental fatigue using a portable terminal device comprising an operation unit, an imaging unit, a display screen, and a recording unit, the method comprising the steps of: 1) measuring ambient light using the imaging unit; 2) displaying a blinking image on the display screen while a flicker frequency of the blinking image is being monotonically changed with time from a start frequency to an ending frequency, 3) recording the flicker frequency in the recording unit, 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; 4) associating first frequency data, which is the measurement frequency measured when the user is specified as being healthy via the operation unit, with first environmental data, which represents the ambient light measured by the imaging unit, and storing the associated data in the recording unit; and 5) calculating 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, to evaluate a degree of fatigue of the user, and displaying the degree of fatigue on the display screen, 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. 1) measuring ambient light using the imaging unit; 2) displaying a blinking image on the display screen while a flicker frequency of the blinking image is being monotonically changed with time from a start frequency to an ending frequency, 3) recording the flicker frequency in the recording unit, 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; 4) associating first frequency data, which is the measurement frequency measured when the user is specified as being healthy via the operation unit, with first environmental data, which represents the ambient light measured by the imaging unit, and storing the associated data in the recording unit; and 5) calculating 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, to evaluate a degree of fatigue of the user, and displaying the degree of fatigue on the display screen, 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.