a mechanical acceleration switch acting in response to a collision involving a vehicle tends to occur with chattering , thereby lowering the reliability of operation due to insufficient application of an ignition current to an igniter of an air bag in the vehicle . when the mechanical acceleration switch is closed and first - stage igniters are ignited , a forced igniter is turned on . even if chattering occurs in the mechanical acceleration switch , the first - stage igniters and a backup capacitor are conductively connected by turning on a driving transistor , such that an electric current is sufficiently delivered to the igniters . as a result , it is possible to improve the reliability of the operation of starting the air bag . furthermore , it is no longer necessary to increase a capacity of the capacitor and change the constitution of the mechanical acceleration switch to decrease chattering therewith .