PURPOSE: To ensure accurate detection even for minute knocking, by comparing the mean value of amplitude of an engine vibration detecting signal for a prescribed period decided on the basis of the time point of ignition.

CONSTITUTION: The pulses given from a crank angle sensor 16 with each 720° crank angle corresponding to a cycle of an engine are applied to an interrupting circuit 40 along with the pulses given from a crank angle sensor at every 30° crank angle. Thus an interruption is applied to a microprocessor 62. As a result, the output of a peak holding circuit 50 for the engine vibration detecting signal produced by a knocking sensor 12 is fetched, and the mean value is calculated for the detecting output of vibration for a prescribed period decided on the basis of the time point of ignition or with a prescribed frequency of ignition and via an MPU62. This mean value is compared with the reference value to decide the presence or absence of knocking. In such a way, even minute knocking is accurately detected to increase the detecting accuracy of knocking.