A shock absorber has a rod that is retractably inserted in an axial direction inside the cylinder, a piston that is provided at a tip of the rod and partitions the inside of the cylinder into an extension-side chamber and a compression-side chamber, a rod guide that is provided at an open end of the cylinder and axially supports the rod, a subcylinder that is provided on the rod guide and forms an annular gap between itself and the cylinder, and a subpiston that is provided on an outer periphery of the rod closer to a base end thereof than the piston, that has an outer periphery in sliding contact with an inner periphery of the cylinder, and that is capable of leaving and entering the annular gap.