A gas generator for an airbag is provided that allows maintaining combustibility as originally designed. An ignition energy generated by an activation of a first igniter 21 flows via a first communication hole 24, a first intermediate chamber 30 and second communication holes 43, into a second combustion chamber 40, to cause a gas generating agent 43 to ignite and burn. The ignition energy flows into the second combustion chamber 40 with making a detour, and therefore, the gas generating agent prevents from broken by the impact and the combustibility thereof is never changed.