1. A method of detecting an occurrence of a knock in an internal combustion engine by detecting an ionic current flowing in a cylinder of the engine from initiation of combustion therein and judging a knock component overlapping the ionic current based on a judging level, the method comprising the steps of: detecting a peak value of the ionic current; and relatively correcting at least one of the judging level and the knock component based on the peak value thus detected so as to make more difficult a judgment of whether the knock occurs or not as the detected peak value increases.