A bolt (10) includes a flange (14) that bulges radially out near a head (12) at one axial end. The flange (14) contacts a second connector (60) at a proper connection to prevent further tightening. A stepped recess (16) is located between the head (12) and a screw portion (13), and is recessed sufficiently to fracture when an excessive tightening torque is applied. A retaining groove (18) is located between the stepped recess (16) and the screw portion (13) and is recessed radially inwardly and receives a retaining ring (80) for preventing detachment of the bolt (10) from a first connector (40) by contacting the first connector (40) before connection. Seal grooves (15) are located between the stepped recess (16) and the flange (14) for receiving seals (30) to be held in close contact with the first connector (40) in a liquid-tight manner at the time of proper connection.