In a misfire detector device for use in internal combustion engine, an electrical interrupter circuit on-off actuates a primary current flowing through a primary circuit of an ignition coil to induce a sparkplug voltage. A check diode is provided in a secondary circuit of the ignition coil to prevent a current flowing back to the ignition coil. The spark plug has a center electrode, a front end of which is projected from an insulator, and an outer surface area of the projected front end being 25 mm² or more. A sparkplug voltage detector circuit detects an attenuation time length of a sparkplug voltage waveform presented subsequent to a time period predetermined after an end of a spark action of the spark plug. On the basis of the attenuation time length, a distinction circuit determines whether a misfire occurs in a cylinder of an internal combustion engine.