PURPOSE: To make light quantity adjustment with a very high response speed by adjusting a light quantity adjusting means so that the light reception power of a light receiving means is below No.1 light reception quantity, and adjusting a light emission power driving means so that the value is below No.2 light reception quantity as the optimum light reception power.

CONSTITUTION: A microcomputer 5 controls an LED bias circuit 21, and a light emitting diode 1 is allowed to make light emission with the max. power, and the light reception power decreasing index is set to an appropriate initial value. It is judged whether the light reception quantity is below No.1 light reception quantity specified previously, and if no, an adjustment is made so as to increase the penetrativeness of the filter part of a light quantity adjusting plate 4. Then the LED bias circuit 21 is controlled to vary the light emission power decreasing index, and the light reception quantity of a light receiving element 3 is adjusted under No.2 light reception quantity as the optimum light reception quantity. Thus the light reception quantity of the element 3 can be controlled very quickly by adjusting the light emission power of the light emitting diode 1 and the light quantity adjusting plate 4.