an combustion state detecting apparatus for an internal - combustion engine is equipped with : an ignition coil for applying a high voltage for igniting to the spark plugs of the cylinders of an internal - combustion engine ; an ionic current detecting circuit for applying a bias voltage to at least one spark plug to detect ionic currents flowing via the spark plug which has just been subjected to ignition control ; comparator circuits which compare detected ionic current signals with threshold values and turn them into ionic current pulses ; and an ecu which drives the ignition coil according to a crank angle signal and determines the combustion state of the internal - combustion engine . the ecu changes the threshold values for each ignition control and judges the combustion state according to the respective threshold values and the state in which the ionic current pulses are generated .