Abstract: A gas spring 20 having a piston 23 slidably disposed within a cylinder 21 sealed with air 25, with one end of the cylinder closed, having a rod 22 fixed to the piston 23, having cylinder grooves 29 extending in an axial direction of the cylinder formed on the inner periphery of the cylinder so that an extension side damping force is generated in an extension process in the cylinder grooves, and having one end of the cylinder and the rod fitted to a vehicle body with the other end fitted to a rear door pivoted on the body, wherein the cylinder grooves are formed in both-side movable areas X and Z excluding a locking area Y in a stroke area of the piston.