A knock determining device of an internal combustion engine comprising: a vibration detecting means for detecting vibration of the internal combustion engine and producing a signal corresponding to the vibration; an intensity computing means for retrieving from the signal vibrational components in a plurality of frequency regions in which peaks of vibration intensity are located when knock occurs; a background-noise computing means for calculating background noise detected as vibration by the vibration detecting means that is caused by factors other than knock; a frequency computing means for determining a number of specific frequency regions from which to determine whether knock is occurring by excluding from the plurality of frequency regions certain frequency regions designated as frequency regions requiring exclusion due to the intensity of false-detection causing noise as a proportion of the background noise of the engine in the certain regions; and a knock determining means for determining the occurrence of knock based on the vibration intensity in the specific frequency regions obtained by excluding the frequency regions requiring exclusion; characterised in that the knock determining means is configured such that the number of specific frequency regions is increased as the false-detection causing noise as a proportion of the background noise is decreased by limiting the frequency regions requiring exclusion to those of the plurality of frequency regions in which the false-detection causing noise causes an intensity of vibration in the background noise in a range that impairs the ability to detect knock. a vibration detecting means for detecting vibration of the internal combustion engine and producing a signal corresponding to the vibration; an intensity computing means for retrieving from the signal vibrational components in a plurality of frequency regions in which peaks of vibration intensity are located when knock occurs; a background-noise computing means for calculating background noise detected as vibration by the vibration detecting means that is caused by factors other than knock; a frequency computing means for determining a number of specific frequency regions from which to determine whether knock is occurring by excluding from the plurality of frequency regions certain frequency regions designated as frequency regions requiring exclusion due to the intensity of false-detection causing noise as a proportion of the background noise of the engine in the certain regions; and a knock determining means for determining the occurrence of knock based on the vibration intensity in the specific frequency regions obtained by excluding the frequency regions requiring exclusion; characterised in that the knock determining means is configured such that the number of specific frequency regions is increased as the false-detection causing noise as a proportion of the background noise is decreased by limiting the frequency regions requiring exclusion to those of the plurality of frequency regions in which the false-detection causing noise causes an intensity of vibration in the background noise in a range that impairs the ability to detect knock.