A dual-type gas generator for inflating air bags is provided in which a first igniter structure having a first combustion chamber accommodating a first gas generating material and a second igniter structure having a second combustion chamber accommodating a second gas generating material both exhaust, when activated, to exhaust gas to a common filter chamber. When the igniter structures are intended to be activated separately and with a time lag, gas generated in the first chamber often flows into the second chamber and may ignite the second gas generating material. This malfunction can be prevented by various flame-transferring preventing structures that preclude direct flow of gas from the first chamber into the second chamber through the common filter chamber.