To provide a portable terminal device changing luminance of a display part for suppressing power consumption without giving any uncomfortable feeling to a user.

The portable terminal device is equipped with: a display part 40 that displays information; an input part 30 that outputs an update signal to update the information displayed on the display part 40; an illuminance sensor part 10 that measures brightness of an ambient environment to output an illuminance detection value (a) indicating brightness of the ambient environment; and a luminance control unit 20 that controls luminance of the display part 40 on the basis of the update signal and the illuminance detection value (a). The luminance control unit 20 is equipped with: a luminance setting unit 22 that sets luminance of the display part 40, and outputs a luminance setting value(b) indicating the luminance set in the display part 40; and a comparison unit 21 that calculates a comparison value between the illuminance detection value(a) and the luminance setting value(b), and determines whether the comparison value is within a preset predetermined value to determine matching between the brightness of the ambient environment and the luminance of the display part 40.