An object of the present invention is to provide a plug/socket assembly in which the plug will never pull out of the socket even if an excessive pull-out force is applied to the plug. A plug/socket assembly (10) comprises a plug (20) and a socket (30) for receiving and fixing the plug (20) therein. The socket (30) includes a socket main body (31) having a plug insertion hole (311), balls (32) which are provided with the socket main body (31) in a radially displaceable manner with respect to the plug insertion hole (311), a sleeve (33) which is mounted over the outer surface of the socket main body (31) and is displaceable along the axial direction of the plug insertion hole (311), and a compression spring (34) for urging the sleeve (33) along the axial direction toward the inlet port (313) of the plug insertion hole. The inner surface of the sleeve is provided with a slide surface (332). The slide surface has first to fourth regions (P1-P4). The fourth region (P4) is shaped such that with an external pull-out force being applied to the plug, even if the balls apply a radially outward force to the slide surface, no leftward force to push back the sleeve will be produced.