A coupling has a socket and a plug that is inserted into and connected to the socket. The socket has a locking element and a sleeve actuating element that are radially displaceable to lock the plug as inserted into the socket. A sleeve is provided over the outer peripheral surface of the socket. The inner peripheral surface of the sleeve has a locking surface that presses the locking element from the radially outer side into a lock position. Further, the inner peripheral surface of the sleeve has an inclined surface extending in the circumferential direction thereof. As the plug is progressively inserted into the socket, the sleeve actuating element is pushed radially outward by the plug while engaging the inclined surface, causing the sleeve to be displaced in the circumferential direction. Thus, the inclined surface radially aligns with the locking element, and the locking element is pushed radially outward while engaging the inclined surface by the plug inserted progressively. When the plug has been inserted into the socket by a predetermined length, the locking element and the sleeve actuating element are received in a groove provided on the outer surface of the plug, resulting in disengagement from the inclined surface. Consequently, the sleeve is returned to the initial position to hold the locking element from the radially outer side, thereby locking the plug in the socket.