PURPOSE: To correctly decide a knocking intensity regardless of the rotary speed of an engine by adding a DC voltage of a proper level and obtain a background noise level.

CONSTITUTION: An amplifier 20 amplifies an averaging signal from an average circuit 6 and at the same time, addes a DC offset voltage Vc of a prearranged level to the said signal. Following this process, the amplifier outputs the said signal to a comparator 8 as a background noise level signal. Then an offset voltage Vc is adjusted so that an output signal from the comparator 8 may have the same pulse number and pulse width against the same knocking intensity in a low rotation region and a high rotation region. Under this constitution, it is possible to accurately decide the same intensity against a knocking of the same intensity regardless of the rotation number of an engine, based on a knocking intensity signal obtained by integrating an output pulse from the comparator.