A rupturable member in which the rupturing manner thereof can be controlled, and a gas generator using the rupturable member are provided. A fragile portion that ruptures preferentially and regulates the rupturing direction of the rupturable member is formed, the rupturable member being provided with a stress dispersion portion formed in a shape or structure to disperse stress when the rupturable member is ruptured, in the tip end of the fragile portion or forward for the rupturing direction of the fragile portion.