PURPOSE: To provide an inflator for inflating an airbag for protecting vehicle occupant, capable of surely controlling the flow rate of a gas flowing from the inflator to the airbag.

CONSTITUTION: An inflator 10 includes a container 14 contg. an amount of gas 12. The container 14 houses an igniter assembly 34 for igniting the gas 12. The inflator 10 has a valve means 40 for controlling the flow rate of the gas 12 from the inflator, and the valve means 40 has a cylinder 42 and a ball piston 52 movably inserted in the cylinder 42 having a gas outlet 50 for directing the gas in the container to the airbag. An elastic member 60 disposed in the cylinder 42 resists a movement of the ball piston due to the gas 12 and controls the ball piston movement so as to control the flow rate of the gas flowing through the gas outlet.