Coupling holding means 50 comprising retainers 53 and retainer engaging portions 52 for holding a coupled state is provided between a socket S10 and a plug P10 of a coupler. Engaging pawl portions 54 are formed on the inner sides of a pair of retainers 53 provided on both sides of a nozzle portion 32. By causing the retainers 53 to move outwardly in a coupling operation, a gap between the retainers 53 and the nozzle portion 32 can be made small. By forming interference preventing portions 58 in the engaging pawl portions 54, the retainers 53 can be provided at a smaller interval than the outer diameter of the nozzle portion 32 whereby the coupler can be made even more compact. Further, by forming the retainers 53 higher than the foremost end portion of the nozzle portion 32, the nozzle portion 32 can be protected by the retainers 53 even in an unexpected event such as falling and damage will not extend directly to the nozzle portion 32.