An output signal of a knock sensor is converted by an A/D conversion part in a specified knock determination range. In a time-frequency analysis part, data of frequency, time, and vibration intensity are extracted at the same time from an output signal of the knock sensor, and the time-varying patterns of vibration intensities in multiple frequency ranges are extracted. A knock determination part computes the number of time-varying patterns of vibration intensity which rise at same time. The knock determination part executes a knock determination based on whether the number of the time-varying patterns of vibration intensity is greater than a knock determination threshold.