A connector-incorporating plug which has a built-in connector in a tube-shaped barrel and in which a coupling is mounted at an outer circumference of the barrel at a front end thereof so as to be movable along a center axis of the barrel includes a C ring mounted, in a detachable manner, in a groove formed on the outer circumference of the barrel and restricting movement of the coupling toward a rear end of the barrel. The C ring is made of a resin material. An outer circumferential side portion of an intermediate portion of a C shape of the C ring protrudes back and forth in the thickness direction thereof to be thick.