Abstract: There is provided a gas spring 20 in which: a cylinder groove 29 is formed on an inner surface of a cylinder 21 slidably arranging a piston 23 therein, the piston 23 being fixed at one end of a rod 22 and the cylinder groove 29 extending in an axial direction of the cylinder throughout the range of piston stroke except an area nearby a stopper 30; and an airlock chamber is definable nearby the stopper of the cylinder so that damping operation can be performed when being fully extended at the end of the extending process, characterized in that the cylinder groove is a square groove having a substantially square cross section with a constant groove width W and a groove depth H gradually reduced toward an area nearby the stopper.