An apparatus for detecting knock for an internal combustion engine having a plurality of cylinders in which a vibration sensor is attached to each cylinder and a peak value of vibration detected by the sensor is sampled over a predetermined range near the top dead center. The sampled value is converted from analog to digital and stored for a predetermined number of sampling times to obtain the average value for each cylinder. Then the frequency of occurrences in which the ratio between the sampled peak value and the average value exceeds a reference value is obtained, and the knock is determined when the frequency of occurrences in any one cylinder exceeds a predetermined value or when the average frequency of occurrences of the all cylinders exceeds another predetermined value.