An combustion state detecting apparatus for an internal-combustion engine is provided for preventing control errors and detection errors by preventing a biasing circuit from discharging at the start of energization, thereby allowing good sensitivity for detecting ionic current to be maintained. The combustion state detecting apparatus comprises an ionic current detecting circuit which includes a biasing circuit connected to the low voltage end of a secondary winding of an ignition coil and which detects ionic current flowing from the biasing circuit via a spark plug after the combustion of a fuel-air mixture. A current limiting circuit is provided between the low voltage end of the secondary winding and the biasing circuit. An electronic control unit detects the combustion state at the spark plug according to the ionic current. The biasing means circuit applies a bias voltage of the opposite polarity from high voltage for ignition to the spark plug via the low voltage end of the secondary winding; and the current limiting means controls the current flowing from the biasing means to the spark plug via the secondary winding, thus restraining the voltage at the high voltage end of the secondary winding when starting current supply to a primary winding.