PURPOSE: To lessen burden on the hardware and reduce the cost by furnishing a means to generate a peak value signal and an averaged signal from the output signal of a knock sensor, setting the knock judging threshold from he averaged signal, and by comparing this with the peak value signal.

CONSTITUTION: The output signal from a knock sensor 1 is fed to a peak hold circuit 11 and an averaging circuit 12, wherein at the circuit 11 a peak value signal is obtained each time a reset signal is impressed from a microcomputer 10A at each ignition, while the other circuit 12 gives an averaged signal accord ing to a sensing signal from the knock sensor 1. These peak value signal and averaged signal are A/D converted to yield respective converted values Vp and VA. These peak value Vp and threshold VTH are compared by a comparing means 16, and the knock control amount is calculated by a delay angle correcting means 9 on the basis of knock judgement and Vp-VTH difference, and this result from calculating shall serve a delay angle correction process. Meantime a fail sensing means 15 senses fail of the sensor 1 on the basis of the signal VA or VTH.