PURPOSE: To obtain a high-accuracy result of measurement, by taking the incident light for compensation into an image pickup device to obtain the compensation coefficient after setting a fixed period during which the incident light is cut off previously and perfectly.

CONSTITUTION: The shutter 4 is closed at the step (St) 11 to erase the after-image which is taken into the image pickup device 3 through the precedent measurement, and the cut-off period is kept for a fixed time (St 12). Then the video signal for compensation is taken in (St 13) without generating the LED chip in order to form the compensation signal. This video signal is partitioned into micro regions and converted into the digital signal with every region (St 14). The total sum of these signals receives an addition (St 15), and the compensation coefficient is decided from the result of addition. This coefficient is compared (St 16) with the reference value which is stored previously. Based on this result, the coefficient α for compensation is decided (St 17).