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 lengths (crank angle, or time period) from a starting point to a latest terminating point of the time-varying patterns of vibration intensity in at least two frequency ranges, which rise at a same time. The knock determination part executes a knock determination based on whether the lengths are greater than a knock determination threshold.