When a user indicates a point on a display screen of a computer using a pen sensor, first the display screen is divided into a plurality of areas and the areas are displayed by mutually different colors or brightnesses. The plurality of colors or brightnesses displayed are scanned by the pen sensor and the colors or brightnesses detected by the light receiver to judge in which area the indicated point is located. Next, the judged area is further divided into a plurality of areas which are displayed by mutually different colors or brightnesses. Next, a routine similar to the above is successively repeated to narrow down the area in which the point indicated by the pen sensor is located and finally determine the position of the point, that is, the XY coordinates on the display screen.