A control system for controlling the ignition of a plasma-jet spark plug provided in an internal combustion engine senses an operating condition of the internal combustion engine, and determines an ignition mode of the plasma-jet spark plug in accordance with the sensed operating condition. The control system performs an ignition control of breaking down the insulation across a spark discharge gap by applying a first electric power to the plasma-jet spark plug, and producing plasma in the vicinity of the spark discharge gap by applying a second electric power to the spark discharge gap in a state of dielectric breakdown. The control system performs this ignition control according to the ignition mode determined as mentioned above.