PURPOSE: To detect knocking more accurately, by extracting not only the fundamental oscillation of the knocking but also higher harmonic components as a signal.

CONSTITUTION: An oscillation pickup 11 is installed in an engine block, and the output of this pickup 11 is inputted to band pass filters 13 and 14 through an amplifier 12. The band pass filter 13 is tuned to the fundamental oscillation frequency of knocking, and the band pass filter 14 is tuned to higher harmonics. Comparators 15 and 16 compare output levels of band pass filters 13 and 14 with specific levels; and if output levels of filters 13 and 14 are higher, the knocking is discriminated, and output signals are made high. An OR circuit 17 operates OR between outputs of comparators 15 and 16; and if one or more input signals are high, the output is made high, and the occurrence of the knocking is discriminated.