A gas spring includes a vertical cylinder, a piston rod extending out of the cylinder through the upper end thereof, a rod guide provided on the upper end of the cylinder for guiding the piston rod, a first seal mounted on the rod guide, a partition wall provided in the cylinder to partition the interior thereof into an upper oil chamber and a lower gas chamber, and a second seal cooperating with the partition wall and slidingly engaging with the piston rod. At least two normally closed first passages are provided in the rod guide and communicate the interior of the cylinder with the outside when filling gas and in filling oil into the cylinder, and a second passage is provided to communicate the gas chamber with the oil chamber when filling gas into the gas chamber.