PURPOSE: To enable the misfire of all the cylinders in an internal combustion engine using a bipolar distributor-less ignition system to be detected properly.

CONSTITUTION: When a high voltage is generated on the secondary side of an ignition coil 26, it is applied to positive and negative spark plugs 52, 54 to cause a spark discharge, and a first capacitor 42 is charged therewith. The first capacitor 42 then applies voltages to the positive and negative spark plugs 52, 54 between which the spark discharge has ceased. If the cylinder is in an ignition cycle and normal combustion has taken place inside the cylinder, the voltage applied to the positive spark plug 52 is attenuated because a current is made to flow between the center electrode and the outside electrode of the positive spark plug 52 by ions generated during the combustion. Based on the attenuation characteristic of the voltage, a misfire detecting circuit 49 detects that the normal combustion has taken place inside the cylinder.