A knock sensor signal 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 intensity of multiple frequency ranges are extracted. Contours of the time-varying patterns of vibration intensity in multiple frequency ranges are extracted. Vectors indicating directions along which the vibration intensities vary are approximated by the contour of the time-varying pattern of vibration intensity in each frequency range according to the least-squares method. A knock determination is executed based on whether directions of any two or more of multiple vectors are within a predetermined range. Alternatively, a knock determination is executed based on whether a ratio of length of any two or more of the vectors is within a predetermined range.