PURPOSE: To accurately decide the adaptability of a light receiving signal at the time of binarizing conversion, by comparing a light receiving level with a level for decision, storing them, and deciding the allowance of the change of the light receiving signal for a reference value based on a stored result.

CONSTITUTION: When the level of the light receiving signal detects an object, no FF29 is set until the completion of the next binarizing operation if no allowance is given on the level of the light receiving signal and the level is not lowered under a lower limit level. Therefore, the output of a level deciding FF34 is inverted to an L level, and alarm is issued. Adversely, when the level of the light receiving signal changes over a lower limit voltage, all of the FFs 27∼29 are set, then, no alarm signal is issued. And assuming that the allowance is given on the level of the light receiving signal and it is lowered and changed under the lower limit level, that is, even when the change of the level of the light receiving signal occurs gradually, the allowance of the level of the light receiving signal is judged based on the stored result stored in a storage means 23 by a decision means 32. As a result, no alarm signal is issued.