PURPOSE: To increase the efficiency of an internal combustion engine and to prevent it from knocking by detecting acoustic emission generated during the operation of the internal combustion engine, and monitoring the level of the AC component of an electrical signal for adjustment of ignition timing.

CONSTITUTION: This controller 14 of an internal combustion engine 12 detects acoustic emission generated during the operation of the internal combustion engine 12 by a converter 16 and generates an output signal meeting the activity of the emission. The output signal is rectified by a rectifier 20 and enveloped by an envelope device 22 to generate an electrical signal. The electrical signal is supplied to a DC component removing device 24, a rectifier 26, a mean level detector 28, and to an analyzer 30 to monitor the mean level value of an AC component. A feedback signal is supplied to the ignition timing device 32 of the internal combustion engine 12 from the analyzer 30 to adjust ignition timing. It is thus possible to increase the efficiency of the internal combustion engine 12 and prevent preignition.