A gas generator for an air bag having a simple structure, which is activated to apply as small an impact as possible to a vehicle occupant at the initial stage of activation and, thereafter, can reliably protect the vehicle occupant by rapidly increasing gas pressure, is provided. It is a gas generator for an air bag in which an inner cylindrical member is disposed in the housing to define an ignition means accommodating chamber and a combustion chamber, plural flame-transferring holes for communicating the both chambers with each other are formed on the circumferential surface of the inner cylindrical member, and the plural flame-transferring holes are formed at a distance of less than 30% of the axial length of the inner cylindrical member, from an end of the inner cylindrical member. A gas generator, which can use even a gas generating agent with less ignitability, and further can exhibit enough operation performance at the initial stage of activation even when using such a gas generating agent, is provided. It is a gas generator for an air bag in which an inner cylindrical member is disposed in the housing to define an ignition means accommodating chamber and a combustion chamber, and the central angle formed by the adjacent communicating portions which are formed on the circumferential surface of the inner cylindrical member is adjusted to be not more than 60 degrees.