To surely prevent oozing of fluid from an auxiliary opening/closing valve in a female joint in a separated condition of both the joints by a simple structure, so that the fluid containing air can be surely sealed even when a residual pressure is left and in the vicinity of an ordinary pressure in the fluid inside the female joint.

This coupling device is provided with a male joint 3 and a female joint 4 connected detachedly to this male joint 3. An annular seal member 78, integrally formed with an annular seal part 78a sealing between a protruded cylinder part 13 of the male joint 3 and a cylindrical member 70 in a connection condition and an annular seal part 78b formed with a valve element part 73 of the cylindrical member 70, is mounted in a tip end part of the cylindrical member 70 of the female joint 4.