A combustion state detecting apparatus for an internal-combustion engine improves the signal-to-noise ratio of an ionic pulse signal to achieve good interfacing characteristic, high detection accuracy, and high control reliability without adding to cost. An electronic control unit (2A) which detects the combustion state in a spark plug according to an ionic pulse signal (Gi) includes: an edge detecting circuit (36) for detecting an end edge of an ionic pulse contained in the ionic pulse signal in a detection zone from a second reference crank angle to a first reference crank angle; a level detecting circuit (37) for detecting the level of the ionic pulse signal at the first reference crank angle; and a circuit (38) for determining the combustion state of the internal-combustion engine according to a detection result (Ni) received from the edge detecting circuit and a detection result (Li) received from the level detecting circuit. Thus, an ionic current detection signal can be pulsed using a simple circuit configuration, and the simple determining logic is used to reduce the load on the arithmetic processor of the electronic control unit.