An output signal of a knock sensor is filtered with a plurality of band-pass filters to extract vibration waveform components of a plurality of frequency bands (f1-f4). Weighting coefficients (G1-G4) which multiply the vibration waveform component of each frequency band are established in such a manner as to be a small value as a noise intensity of each frequency band becomes larger. Thereby, the vibration waveform component of a plurality of frequency bands is synthesized by weighting according to an influence of a noise intensity of each frequency band. Even when the noise is superimposed on the vibration waveform component of any of the frequency bands, it becomes possible to reduce the influence of the noise and to synthesize the vibration waveform component of each frequency band, and an accurate knock determination can be performed based on the composite vibration waveform.