To reduce the impact at full extension of a piston by soften the braking of a piston when it is fully extended.

A gas spring 20 is constituted by executing braking in an extension stroke and at full extension time in a manner wherein a piston 23 is secured to one end of a rod 22, the piston 23 is slidably arranged in a cylinder 21, in its internal periphery, a cylinder groove 29 extended in an axial direction of the cylinder is formed in a stroke region of the piston except in the vicinity of a stopper part 30, further an air lock chamber can be formed in the vicinity of the stopper part of the cylinder. Here, the cylinder groove is formed in an angular groove of almost quadrangular section having a fixed groove width to gradually decrease a groove depth toward in the vicinity of the stopper part.