PURPOSE: To make it possible to detect only the abnormality of an actual circuit, by inputting a part of a transmission signal to a circuit for receiving the reflected wave from an object to apply processing thereto and detecting whether the signal after processing is generated.

CONSTITUTION: The transmission signal A from an AND circuit 4 is attenuated by a resistance voltage dividing circuit 14 to obtain a signal E which is, in turn, inputted to a comparing circuit 16 along with the signal C, which is obtained by receiving the reflected wave from an object 5, through an amplifier 7 and both signals are respectively compared with predetermined levels to input the wave form shaping signal Pc, Pe of the signals C, E to a masking circuit 15 and a circuit abnormality discrimination circuit 30. In the circuit 30, AND logic of the signals Pc, Pe and the output M of a monostable multivibrator 11 is taken to output a pulse signal K at every input of the signal Pe and the normal and abnormal operations of the amplifier 7, the rectifier circuit 8 and the circuit 16 are judged by the presence or absence of the generation of the signal K.