A method of determining a charging capacitance of a capacitor in an air bag system having an electronic control unit for controlling a gas generator, a bus line having a loop wire extending from the electronic control unit, gas generators connected to the loop wire, each gas generator having an igniter connected to the loop wire via a connector, one of the igniter and the connector being provided with an integrated circuit having the capacitor for providing power to a heat generating portion, a switching circuit for turning ON/OFF a supplying of current to the heat generating portion, and a microcomputer unit for controlling the switching circuit, the method including, determining a minimum charging capacitance of the capacitor necessary to operate the igniter based on a charging voltage and a minimum ignition energy required for making the heat generating portion of the igniter generate heat to ignite the priming.