PURPOSE: To take out only vibrations near the frequency of vibrations caused by knocking without using a band-pass filter, by using a disc vibrator and making the same resonate with the frequency of knocking.

CONSTITUTION: The detector is fitted to the main body of an engine by the screw part 1a of a housing body 1 and, therefore, the vibrations of the engine at the time of knocking thereof are transmitted to the housing body 1 at the time of detection. Since the flexible disc vibrator is fixed to and supported by the convex part 1c of the housing body, by welding or the like, at a place other than a node point, the convex part 1c serves as a driving point for the flexible resonance of the disc vibrator 2. The flexible disc vibrator 2 is prepared to have a frequency nearly equal to the frequency of vibration at the time of knocking. A mechanical-electrical transducer 3 bonded to the disc vibrator is given distortion and generates an electric signal thereby, which is taken out for an outside electric apparatus from a terminal 6 formed of a conductor through lead wires 7a and 7b.