In determining a knock of an engine (11), a vibration intensity is calculated by logarithm-transforming a peak value or an integral value of an output signal of a knock sensor (28). Further, a central value VMED of a vibration intensity distribution is calculated, and also a standard deviation σ in a region where the vibration intensity is smaller than this central value VMED is calculated. A vibration intensity reference value VIB is calculated as VMED - u x σ, thereby setting the vibration intensity reference value VIB to be close to a minimum of the vibration intensity distribution. Further, a knock reference value KCK is set by adding a predetermined value K to the vibration intensity reference value VIB. The vibration intensity detected by the knock sensor (28) is compared with the knock reference value KCK to determine presence/absence of a knocking.