PURPOSE: To prevent erroneous output even if either of outputs of an input side equipment is equal to or lower than a normal output range due to failure by providing a selection circuit at the preceding stage of a differential amplification circuit and selecting a signal whose level is the highes out of signals over a specified level.

CONSTITUTION: The output of a power transducer 1 and that of a constant power supply circuit 10 are input to a HIGH selection circuit 8, thus selecting the output of the transducer 1 and feeding it to a differential amplification circuit 3. When the level at the input side which receives the output of the transducer 1 reaches 0 level due to failure of the transducer 1, etc., a signal at a specified level from the constant power supply circuit 10 is selected and is fed to the differential amplification circuit 3. When the differential amplification circuit 3 receives a signal which is selected by the HIGH selection circuit 8, it performs operation judging that the output of the transducer 1 is at 0 level. The signal at the specified level which is selected by the HIGH selection circuit 8 is supplied also to an alarm output circuit 9 and the alarm output circuit 9 outputs the alarm signal to the outside.