PURPOSE: To improve a device which detects knock with high precision in each of a plurality of frequency bands of the vibration with a plurality of resonance frequencies which are characteristic the respective engines, when knock is gererated in the engine.

CONSTITUTION: In a knock detecting device for an internal combustion engine which detects the knock generated in the interrnal combustion engine, a knock sensor 8 for detecting the vibration of the internal combustion engine is installed, and a plurality of filter means 5 separate the detected vibration to a plurzlity of frequency bands, and extracts them, and a knock existence judging means 61 detects the vibration in each of a plurality of frequency bands, and judges the existence of the knock, and a knock frequency judging means 62 calculates the knock judgement frequency in each frequency band, and a synthesized knock judging means 63 performs the final knock judgement on the basis of the knock judgement frequency.