PURPOSE: To prevent knock controllability from deteriorating by performing a knock suppressing control in each cylinder normally being based on a knock discrimination signal and limiting reflection by the knock discrimination signal when it is decided to be error discrimination by a noise.

CONSTITUTION: A digital signal VP, based on a peak level of a knock sensor output signal, is compared with a peak threshold value VTH calculated by a peak threshold value arithmetic part 41 from a signal VA in which an averaged signal of the knock sensor output signal is digitalized, and a deviation level V is subtracted by a subtraction part 42. A delay control angle θR for reflecting a cylinder ignition position to a delay timing side being based on this V is obtained by a delay timing reflection processing part 43. A noise discriminating part 5 for discriminating the deviation level V in misdiscrimination by a noise in each cylinder, being based on a delay timing amount θR per ignition, is provided. Reflection by the deviation level V is limited by generating a delay timing semicircle limit signal VC from a delay timing reflection limiting part 6 being based on a noise discriminating signal VK output from this discriminating part 5.