In order to measure a scanning light cut-off region, an output of a light receiving element and a threshold value set by an MPU are compared by a comparator and a region where the former is smaller than the latter is measured as the scanning light cut-off region. During one cycle of optical scanning, this threshold value is switched in a plurality of stages according to the scanning angle. By accurately measuring the scanning light cut-off region, the correct position and size of an indicator such as a finger or pen are calculated.