Abstract: A pipe fitting capable of minimizing flow resistance, decreasing force required for connection and preventing leakage of fluid therethrough. An operating sleeve slidably mounted on a main member of a socket body is forcedly thrusted and a connecting member of a connecting plug is fitted in an internal passage of the main member while pushing away balls fitted in the main member, so that the connecting member forcedly pushes an on-off valve arranged in the main member to pivotally move it for opening it. Releasing of the operating sleeve from its thrusted position causes the balls to be fitted in an annular groove of the connecting plug, to thereby accomplish connection between the socket body and the connecting plug.