one of the power transistors 41 to 43 has been short circuited to have become always in a conductive state for example a current equal to or higher than a predetermined level of current always flows to the current detecting resistor 85 and therefore the current limiting circuit 64 operates to set all the power transistors 41 to 43 to a cut off state in this state no current flows to any one of the ignition coils 14 to 16 so that it becomes impossible to fire a mixed gas to be supplied to the internal combustion engine thus the internal combustion engine is placed in a position to completely stop its operation however according to the present embodiment as described above when an excess current has flown to the power transistors 41 to 43 the lead frame any one of the lead frames 51 to 53 corresponding to the power transistors 41 to 43 is burnt down therefore a current is not supplied from the short circuited power transistor one of 41 to 43 to the current detecting resistor 85 the short circuiting of one of the power transistors 41 to 43 does not interfere the operation of the rest of the normal power transistors 41 to 43 as described above according to the present embodiment even if any one of.