PURPOSE: To improve the knock detecting accuracy, by furnishing plural knock discriminators, and arranging an exclusive power circuit combined to each of the discriminators, in the system with the knock discriminator to detect selectively the generation of knocking from the output of knock sensors.

CONSTITUTION: Knock discriminators 40 and 50 are provided to input the output signals of acceleration sensors 41 and 51 which are installed at an engine and detect the vibrating acceleration of the engine. The knock discriminators 40 and 50 are composed of frequency filters 42 and 52, analog gates 43 and 53, noise level detectors 45 and 55, and comparators 46 and 56 respectively, and a constant voltage is fed through power supply circuit 47 and 57 connected to a battery 60 respectively. Since the detected output of the acceleration sensors 41 and 51 are of different amplitudes and phases each other, and the operating wave forms of parts of the discriminators 40 and 50 are also different, the variation of consuming currents is liable to give influence each other. However, the power supply circuits 47 and 57 installed respectively to each discriminator 40 and 50 prevent such an influence.