1. A knock determination device for an internal combustion engine, comprising: a detecting unit detecting vibration of the internal combustion engine; and a determination unit connected to said detecting unit; wherein said determination unit includes a plurality of band-pass filters extracting, from said detected vibration, vibrations of a plurality of predetermined frequency bands corresponding to knocking, respectively, an integrating unit calculating a plurality of integrated values corresponding to intensities of the extracted vibrations of the plurality of frequency bands, respectively, and, a synthesizing unit changing a plurality of weights corresponding to the plurality of frequency bands, respectively, such that, among the, plurality of frequency bands, weight on the intensity of vibration in the frequency band of first tangential mode is made smaller than weight on intensities of vibrations in other frequency bands and synthesizing a vibration waveform of a predetermined crank angle interval based on said plurality of integrated values and said plurality of weights on intensities, and, said determination unit determines whether or not knock has occurred in said internal combustion engine, using said synthesized vibration waveform.