A knocking determination device for an internal combustion engine comprising: an acceleration sensor or an in-cylinder pressure sensor mounted to each of cylinder units of an internal combustion engine and configured to measure acceleration or in-cylinder pressure correlated to knocking intensity; and control means for performing knocking determination and control based on a sensor signal measured by the acceleration sensor or the in-cylinder pressure sensor, wherein the control means extracts knocking-frequency waveform signal from first time periods in the sensor signal and computes a first computation value from the knocking-frequency waveform signal, extracts a reference-frequency waveform signal from second time periods in the sensor signal different from the first time periods, computes a second computation value from the reference-frequency waveform signal, and obtains an average of a plurality of the second computation values, obtains a ratio by dividing the first computation value by the average, defines in advance a plurality of sets of a threshold for the ratio and a predefined number of times depending on the threshold, the threshold and the predefined number of times differing between the plurality of sets such that the predefined number of times decreases as the threshold increases, obtains a percentage by dividing the number of times the ratio exceeds the threshold by the predefined number of times, uses the percentage as a knock index for performing the knocking determination, and performs the knocking determination based on a magnitude of the knock index. an acceleration sensor or an in-cylinder pressure sensor mounted to each of cylinder units of an internal combustion engine and configured to measure acceleration or in-cylinder pressure correlated to knocking intensity; and control means for performing knocking determination and control based on a sensor signal measured by the acceleration sensor or the in-cylinder pressure sensor, wherein the control means extracts knocking-frequency waveform signal from first time periods in the sensor signal and computes a first computation value from the knocking-frequency waveform signal, extracts a reference-frequency waveform signal from second time periods in the sensor signal different from the first time periods, computes a second computation value from the reference-frequency waveform signal, and obtains an average of a plurality of the second computation values, obtains a ratio by dividing the first computation value by the average, defines in advance a plurality of sets of a threshold for the ratio and a predefined number of times depending on the threshold, the threshold and the predefined number of times differing between the plurality of sets such that the predefined number of times decreases as the threshold increases, obtains a percentage by dividing the number of times the ratio exceeds the threshold by the predefined number of times, uses the percentage as a knock index for performing the knocking determination, and performs the knocking determination based on a magnitude of the knock index. extracts knocking-frequency waveform signal from first time periods in the sensor signal and computes a first computation value from the knocking-frequency waveform signal, extracts a reference-frequency waveform signal from second time periods in the sensor signal different from the first time periods, computes a second computation value from the reference-frequency waveform signal, and obtains an average of a plurality of the second computation values, obtains a ratio by dividing the first computation value by the average, defines in advance a plurality of sets of a threshold for the ratio and a predefined number of times depending on the threshold, the threshold and the predefined number of times differing between the plurality of sets such that the predefined number of times decreases as the threshold increases, obtains a percentage by dividing the number of times the ratio exceeds the threshold by the predefined number of times, uses the percentage as a knock index for performing the knocking determination, and performs the knocking determination based on a magnitude of the knock index.