A coupler having a socket with a valve and an element for energizing the valve in a closing direction, and a plug having a valve and an element for energizing the valve of the plug in a closing direction, where the plug is detachably fitted and coupled with the socket and is capable of communicating with the socket by opening the valve of the plug and the valve of the socket in the fitted and coupled state,A key groove or a key is provided in either the socket or the plug, and a ring member having a key or a key groove which can be fitted with the key groove or the key is provided in the other of the socket and the plug and the ring member is rotatable about a central axis of connection of the socket and the plug.