Abstract: A gas spring and a method for filling a gas chamber of a gas spring does not require the use of a gas filling valve. Gas spring (5) includes a rod member (20), a cylinder main body (21), a gas filling chamber (22), first and second sealing members (23, 24), a plurality of pin members (25), and a plurality of slots (26). A gas filling implement (30) makes it possible to fill the gas filling chamber (22) with compressed gas in a simple and reliable manner without providing the gas spring (5) with a gas filling valve having a nonreturn valve. Consequently, the entire gas spring (5) is configured symmetrically about the axial center of the rod member (20), and its center of gravity is aligned with the axial center of the rod member (20), making it possible to prevent the gas spring (5) from inducing vibrations in the main shaft.