A knock control apparatus for suppressing knock event in an internal combustion engine by correcting a control quantity such as ignition timing for the engine while evading erroneous knock decision and erroneous knock control when a spark plug of the engine is in a sooting state. The apparatus includes a means (10) for detecting an ion current (i) flowing by way of the spark plug (8) upon combustion of air-fuel mixture within an engine cylinder being monitored, a filter means (14) for extracting a knock signal (Kj) from an output signal (Ei) of the ion current detecting means (10), a knock decision means (23) for deciding occurrence of a knock event in the engine on the basis of the knock signal (Kj), a control quantity correcting means (24) for correcting the control quantity for the internal combustion engine so as to suppress the knock event on the basis of result (H) of decision, and a sooting state detecting means (25) for detecting a sooting state of the spark plug (8) on the basis of a current signal (Gi) detected through the ion current detecting means (10) during a period in which detection of the ion current (i) is not performed, wherein the sooting state detecting means (25) generates a correction inhibiting signal (M1; M2) for inhibiting correction of the control quantity as performed by the control quantity correcting means (24) on the basis of the decision result (H) the sooting state of the spark plug (8) is detected.