It is intended to provide a piston type gas compressor capable of replacing a seal ring of a piston with a new one while preventing incorporation of metallic particles into a processing gas. A gas suction port 9 and a gas discharge port 10 are formed in a flange 2 of a cylinder 3 in communication with a gas compressing space H. With a plug 7 removed from an internally threaded hole 6A, a free piston 5, together with a seal ring 4, can be inserted into and removed from the cylinder 3 through the internally threaded hole 6A.