A portable electronic device includes a foldable housing; a display; an illuminance sensor; a state detection sensor; a memory; and a processor. Based on data received from the state detection sensor, the portable electronic device is recognized to be in the folded state. Responsive to the portable electronic device being in a calibration trigger state which includes the folded state, a first image is displayed in a sensor area of a first display area located on the illuminance sensor, and a second image is displayed in an area of the second display area facing the sensor area. An illuminance value is calculated based on data received from the illuminance sensor while the first image and the second image are displayed, and then compared to a reference value stored in the memory to calculate a calibration value for calibrating measured illuminance values of the illuminance sensor.