PURPOSE: To evaluate the quality of a photo-electric sensor by timely measuring the amount of electric current related to the output change of the photo-electric conversion element.

CONSTITUTION: Closing a checking switch 12, the variable resistor 17 of a measuring instrument 15 is adjusted to control the amount of electric current to be supplied to a test light source 3. Specifically, with the illumination of the light source increased, a detection circuit 8 reads out the D.C. current value when a common terminal COM and a normally open terminal NO are short-circuited, marking the intersection between the days elapsed and the D.C. current value with a circle. Then, reducing the illumination of the test light source 3, the D.C. current value is read out when the circuit connecting the terminals COM and NO is closed, marking the intersection with the days elapsed with a cross. Like measurement of D.C. current is carried out on the 10th day counting from tis marking, again marking the intersections with the days elapsed with a circle and a cross. By tying the circles and crosses timely change of the photo-electric sensor is estimated enabling evaluation if the sensor properly works or not.