PURPOSE: To surely detect misfiring, by recognizing generation of the misfiring when a voltage waveform appears to be in the same phase at each end of a primary winding, in the means such that voltage in the reverse direction to each other from the middle point of the primary winding is applied at a different timing so as to generate an ignition spark in the both positive and negative directions.

CONSTITUTION: A switching transistor Tr14 or 16 is turned on and off by the output of a driver circuit 20, and high voltage in the negative or positive direction is generated in the secondary winding 10c of an ignition coil 10 and applied to ignition plugs 18dW18f or 18aW18c through a distributor 36 or 38. At operation of normal ignition, when, for instance, the transistor Tr14 is turned off, generation of negative voltage in the collector of the transistor Tr16 is prevented by diodes 34, 40 inserted between the transistors Tr14, Tr16 and each primary winding 10a, 10b. On the contrary, at generation of misfiring, when the transistor Tr14 is turned off, positive high voltage is generated in the collector of the transistor Tr16, causing monostable circuits 26, 28 to be triggered, thus a misfiring detecting signal is output through an AND circuit 30.