Abstract: Knocking detecting in an internal combustion engine uses, a vibration sensor detecting engine vibrations; and insert for taking-in output vibration signals from the vibration sensor in a plurality of intervals in a time sequential manner during every explosion stroke of the internal combustion engine; performing a digital frequency analysis on the taken-in output vibration signals in the respective sequential intervals; setting a main resonance frequency necessary for knocking occurrence judgement based upon the instant operating condition of the internal combustion engine; calculating knocking indexes for the digital frequency-analyzed output vibration signals in the respective sequential intervals based upon the set main resonance frequency component; comparing the largest knocking index calculated with a predetermined threshold value determined by a background vibration level and a slice level thereof to determine occurrence of knocking; and renewing the respective background vibration levels using the cal