PURPOSE: To detect knocking highly accurately, by comparing an output signal obtained by normalizing the correlation value of combustion vibration at the time of knocking with a reference signal, based on the correlation value of combustion vibration energy at the time knocking is not present, thereby judging the occurrence of the knocking.

CONSTITUTION: A charge signal corresponding to the cylinder pressure from a cylinder pressure sensor 21 is converted into a voltage signal by a charge amplifier 31. The signal is inputted to first and second converting means 33 and 34 through a bandpass filter 32. Meanwhile, a reference signals S2 and a position signal S3 at every crank angle of 120° and at every 1° from a crank angle sensor 13 are inputted to an integration starting and stopping instruction circuit 35A through an ignition time control circuit 35H. An integration time instruction signal is imparted to converting means 33 and 34. Correlation (integration) amount signals S6 and S7 of combustion pressure vibration energy at the times the knocking occurs and does not occur are inputted to the first and second correcting parts 35B and 35C, and the integrated values are corrected. A signal S7/S6 normalized by a dividing part 35D is compared with a reference signal from a reference value generating part 35F. Thus whether the knocking is yielded or not is judged.