A device performs ambient light sensing using an image sensor. The device includes an image sensor and a receiver configured to receive image data from the image sensor. The device also includes a luminance calculator configured to calculate a code corresponding to the luminance value of the image data based on the image data. The device further includes an image sensor controller configured to change the setup condition of the image sensor in response to the code having a minimum value or a maximum value, among values capable of being assumed by the code. The device additionally includes a brightness measurer configured to output a brightness value in the vicinity of the image sensor, which is identified using the changed setup condition and the code.