An abnormal combustion detection apparatus detects an abnormal combustion of an internal combustion based on a vibration waveform from a vibration sensor attached to the internal combustion engine. A gain circuit of the apparatus amplifies or attenuates the vibration waveform from the vibration sensor, an AD conversion circuit converts a waveform provided by the gain circuit to digital data, and a gain switching unit switches a gain of the gain circuit from a small gain value to a large gain value at a gain switch timing. The gain switching unit separates a large abnormal combustion from a small abnormal combustion in an abnormal combustion determination period for maximizing a dynamic range of an AD conversion circuit and for securely detecting a change of waveform amplitude in case of gain switching in an abnormal combustion detection operation.