PURPOSE: To detect the average position of a substance automatically by calculating the average value of respective address data indicating the pair of a light source corresponding to an optical detection signal with its photodetecting element as the average address of the substance.

CONSTITUTION: When the substance is arranged between two pairs of light emitting diodes and phototransistors and an address reading signal by one light emitting diode and phototransistor is outputted, the counted value of a counter 25 is set up to "1" and an adder 24 is set up to adding status. When an address reading signal by the other light emitting diode and phototransistor is outputted, the counted value of the counter 25 is turned to "2" and the adder 24 in the adding status adds prescribed address data and applies the added value to a divider 28. The divider 28 divides the added result by the counted value 2 and sends the divided result as the average address data. Thus, the average position of the substance can be automatically detected.