PURPOSE: To predict a check time at an early stage by allowing two comparison discrimination circuits to set a reference level of an incident light on one hand and to set a light shield reference level on the other hand, to compare the reference level and a light receiving signal and to discriminate the result.

CONSTITUTION: A comparison discrimination circuit (e) outputs a discrimination output as to whether or not a light receiving signal exceeds a lower limit light receiving level and reaches a stable light receiving level. A comparison discrimination circuit e' outputs a discrimination output when a light receiving signal level is less than an upper limit light shield level almost in the complete light shield. Outputs of the circuits e,e' are inputted to an OR circuit (f), its output is inputted respectively to a fault output terminal (i) via an output circuit (h) and a safety display light 3 via a lighting circuit (g) respectively. That is, when the light receiving signal exceeds a reference level of the circuit (e), a stable incident light is set and when the light receiving level is less than the reference level of the circuit e', stable light shielding is set, and when the level between them, it is discriminated to be in the unstable state and the normality or the fault is discriminated.