PURPOSE: To prevent the occurrence of the influence of noise and to detect the accurate combustion state of a gasoline engine, in detection of a voltage change between the electrodes of an ignition plug.

CONSTITUTION: The passage of the maximum value of a sensor 50 to detect a voltage between the electrodes of an ignition plug after spark discharge of the ignition plug is detected by a maximum value passage detecting part 21 and after a lapse of a given time starting from the detection, reset of a peak hold circuit 35 is released. In noise of an ignition device, since the damping characteristics of the noise are decided according to the characteristics of an ignition coil, by properly setting a given time, the detected voltage of the sensor 50 is held right after damping of the noise. Since the reference voltage of a following voltage change is set based on the held voltage, an accurate voltage change is detectable.