A misfire detecting device for an internal combustion engine including an ion current detecting means for detecting ion currents generated between electrodes of spark plugs; an ion current judging means for judging whether or not the ion current exists at each ignition cycle based on the ion current signal; a misfire judging means judging when no ion current is judged to exist based on an ion current judgment signal; an ignition coil; an ignition coil signal judging means for judging whether or not the ignition coil signal generated in synchronism with an ignition timing exists; and a failure judging means for judging whether or not an failure of the ion current detecting means occurs based on a result from the ignition coil signal judging means when the ion current signal is not detected.