A coupler 10 comprises a socket S10 comprising a valve 16 and a metal helical spring 17 for energizing the valve in a closing direction and a plug comprising a valve 24 and a metal helical spring 25 for energizing the valve in a closing direction and being capable of being detachably fitted and coupled to the socket and being capable of opening the two valves to communicate the socket and the plug to each other in a fitted and coupled state. Elastic member holders 18 and 26 are mounted at an base end portion of the valves 16 and 25 in a sealable manner and in such a manner that the valves can reciprocate in closing and opening directions, said elastic member holders housing the energizing means for energizing the valves and flow paths being formed outside of the elastic member holders. By this arrangement, by housing the metal helical springs in the elastic member holders, contact between the metal helical springs and liquid can be prevented.