PURPOSE: To prevent the detecting operation from being stopped even when a long noise goes in by outputting a signal only when pulse width of plural signals whose level is detected is a set value or below, and outputting a detecting signal when one of signals exist in a set gate.

CONSTITUTION: For instance, a receiving amplifying signal is decided 26 by two threshold levels and a digital signal is generated, and its signal width is detected 27. When the signal width is a set value or below, a differentiating signal is outputted by a terminal part of the signal. Subsequently, in a signal shaping part 28, OR is taken, and a new receiving converting signal outputted to a detecting signal generating part 29, and a signal to a period oscillating part 20 are generated. In this regard, it is allowable that these two signals are the same. When the receiving converting signal exists in a gate of a detecting gate signal generated by a gate signal generating part 30, the generating part 29 drives an output circuit 31. Accordingly, in the case continuous noises exist, even if the noise is brought to level decision 26, it is not outputted as a detecting signal since it exceeds the set signal width, therefore, it does not occur that the detecting operation is stopped. Also, a malfunction caused by the noise is also prevented.