24 is inserted in place of the half wave rectification circuit 11 and the integrating circuit 14 in the fault detecting circuit 10 said electric signal load circuit 24 comprises a constant voltage circuit 25 and a resistance 26 which is serially interposed between the constant voltage circuit 25 and the knocking sensor 8 said resistance 26 is connected at one side of the constant voltage circuit to the abnormality level setting circuit 16 and at the other side of the knocking sensor to the comparison circuit 15 the fault of the knocking sensor 8 is detected in the following way if an output voltage from the constant voltage circuit 25 is applied to an internal resistance 81 in the knocking sensor 8 through the resistance 26 then a voltage value divided to the resistance 26 and the internal resistance 81 is generated in a knocking sensor signal wire 100 said voltage value is fed to the comparison circuit 15 and compared with a voltage value fed from the abnormality level setting circuit 16 in the comparison circuit 15 when the voltage value fed from the abnormality level setting circuit 16 is lower than the divided voltage value fed to the comparison circuit 15 it is decided that the knocking sensor 8 is out of order an output is fed from the comparison circuit 15 of the fault detecting circuit.