In a misfire detector for use in an internal combustion engine having an ignition coil, an electrical interrupter circuit interrupts a primary current flowing through a primary circuit of the ignition coil by which a spark plug is energized. A voltage divider circuit detects a divided voltage of a spark plug voltage applied across the spark plug. A spark plug voltage detector circuit is provided to detect an attenuation characteristic of a spark plug voltage waveform presented subsequent to a time period predetermined either during a spark action of the spark plug or after an end of the spark action of the spark plug. A peak hold circuit is provided to hold a peak voltage of the spark plug voltage waveform presented after the end of the spark action of the spark plug, so that a distinction circuit determines a misfire on the basis of a peak voltage level or the attenuation characteristics of the spark plug voltage waveform.