An ignition range detector for an internal combustion engine is disclosed, in which at each combustion of the internal combustion engine, a pressure signal of a pressure sensor for detecting the pressure in a cylinder is integrated by a signal associated with the crank angle to produce an area S.sub.1 +S.sub.2 presenting the combustion state. At the same time, the pressure signal for a hypothetical case of misfire under the same condition is integrated to produce an area S.sub.1. It is decided whether the ratio (S.sub.1 +S.sub.2)/S.sub.1 is higher than a predetermined value .alpha. or not thereby to detect an ignition range always stably regardless of the operating conditions of the internal combustion engine including the load thereon.