A power circuit, which supplies plasma energy to a spark plug, includes a DC/DC converter which charges a tank capacitor, a voltage limiting circuit which restricts an output voltage of the converter to a predetermined value, a PJ capacitor which is connected to the output side of the converter and is charged by the tank capacitor, and a high breakdown voltage switch which is connected between the PJ capacitor and the DC/DC converter and controls a charging time period of the PJ capacitor in response to operating conditions of an internal combustion engine; and the power circuit switches a voltage limiting value of the tank capacitor for charging the PJ capacitor in synchronization with a driving signal of the high breakdown voltage switch.