A gas pressure actuator includes a launcher, a gas generator that is disposed at a first axial end of the launcher, a rocket part placed inside the launcher, an end pad and a sealing gasket respectively disposed at a second axial end of the launcher, and a gasket holder supporting the gasket. The rocket part includes a first portion disposed at a location corresponding to the first axial end and a second portion that has a diameter less than that of the first portion and that ranges from the first portion toward the second axial end. The end pad includes a position determination part that determines positioning of the rocket part and restricts the rocket part from being displaced in a direction away from the first axial end. The gasket holder may be pressed by the first portion when the rocket part protrudes from the launcher and plastically deformed.