Provided is a control apparatus for an internal combustion engine with which temporal and frequency resolutions necessary for analysis of a frequency of knock by digital signal processing are ensured with limited amounts of processing and memory. The control apparatus includes a lowpass filter for eliminating a high frequency component of an output signal from a knock sensor or a cylinder internal pressure sensor, an A/D conversion unit for performing A/D conversion on the output signal from the filter, and DFT processing units for performing digital signal processing on data obtained by the A/D conversion as knock detection units, each of the DFT processing units includes observation interval setting units for dividing the A/D-converted data to set observation intervals each containing a predetermined number of samples, and a multi-frequency simultaneous analysis unit for simultaneously analyzing multiple frequencies by a discrete Fourier transform for each of the set observation intervals.