Abstract: A knock detecting device for modifying the ignition timing of individual cylinders of an engine, by using averaging circuits which are the same in number as the cylinders, a first distributing switch that distributes the output from a sensor detecting engine vibration to said averaging circuits, a second distributing switch selecting the outputs from the averaging circuits, a detecting means detecting the presence of knocking at each cylinder through comparison of the output from the sensor and the selected output from the averaging circuits, and a control means controlling the ignition timing of the individual cylinders in response to the output from the detecting means.