PURPOSE: To obtain the long life for an ignition plug by transmitting the output of timing signal generators to two ignition circuits at a low load to ignite doubly by a double ignition plug and operating two ignition plugs alternately at a medium load.

CONSTITUTION: Two ignition plug 3 are provided on one cylinder. Each ignition plug 3 is controlled for operation by timing signal generators 6 and 6 via a transmission signal switching unit 5 and ignition circuits 4 and 4. The transmission signal switching unit 5 is provided with a simultaneous transmission circuit 7 to transmit a timing signal to each ignition plug 3 at its every input, an alternate transmission circuit 8 to transmit the timing signal to each ignition plug 3 alternately, and a switch device 9 to switch over two circuits 7 and 8 selectively between generators 6 and circuits 4. The switch device 9 is controlled for switching on the basis of the output of a load-condition detecting device 10 via a control circuit 11. In this construction, each ignition plug 3 is operated as designed to economize the cost of fuel and prolong the life of the ignition plug 3.