PURPOSE: To simplify the hardware constitution and to increase the accuracy and reliability for the detection of knocking, by integrating the engine vibration detecting signals in the different prescribed ranges of crank angles with different time constants and then comparing the levels of said detecting signals.

CONSTITUTION: An interruption is applied to the processing operation carried out by a microprocessor 62 via an interrupting circuit 40 and by the pulse output produced by the crank angle sensors 16 and 18 with each prescribed crank angle. Thus the integral value is fetched by a knocking sensor 12 for the prescribe period before and after the dead point of at least a cylinder which is obtained by an integral network 50 of the engine vibration detecting output. While the integral time constant of the network 50 is controlled by the processor 62 so that the period after the dead point is longer than the period before the dead point. The background signal in a period free from the knocking obtained by the sensor 12 has the good responding performance and high stability without any effect of noise, etc. Then the levels are compared with each other for the integral value between both periods. In such a way, the hardware constitution is simplified to ensure the detection of knocking with high accuracy and high reliability.