A combustion state detecting apparatus for an internal combustion engine is capable of ensuring a satisfactory ion current detection sensitivity and reliability by preventing discharge of bias voltage to an ignition coil to thereby protect the bias voltage against lowering. The apparatus includes an ignition coil for generating a firing voltage, spark plugs to which the firing high-voltage is applied via a high-voltage supply circuitry connected to an output terminal of the ignition coil, a biasing device connected to the high-voltage supply circuitry for applying a bias voltage to the spark plug, a bias voltage protection device inserted between the output terminal of the ignition coil and the biasing device, an ion current detecting device for detecting ions generated in succession to discharging of the spark plug upon application of the firing high-voltage as an ion current which flows through the spark plug under the bias voltage applied to the spark plug, and an electronic control unit for detecting combustion state on the basis of detected value of the ion current.