A connector connecting bolt which is mountable in a first connector and connectable to a mating second connector, comprising: a head portion at or near a first axial end part; a screw portion at or near a second axial end part; a flange portion which bulges radially outwardly near and adjacent to the head portion and can come into contact with the second connector at the time of proper connection to prevent any further tightening of the bolt; at least one stepped recess which is located closer to the first axial end part than the screw portion, recessed radially inwardly and which can be fractured when an excessive tightening torque is applied; and at least one retaining groove which is located between the stepped recess and the screw portion in an axial direction and recessed radially inwardly and into which a retaining member for preventing the detachment of the bolt from the first connector by coming into contact with the first connector before connection is to be fitted, characterized by at least one seal groove which is located between the stepped recess and the flange portion in the axial direction and recessed radially inwardly and into which at least one seal member to be held in close contact with the first connector in a fluid-tight manner at the time of proper connection is to be fitted, wherein the thickness of the flange portion is larger than the width of the seal groove, wherein a width and depth of the seal groove is so set that a shaft portion leading end of another connector connecting bolt does not come into contact with a groove surface of the seal groove of the bolt even if the other bolt faces in an opposite direction relative to the bolt and interferes in an inclined posture, and wherein a thickness of the flange portion and a width and depth of the seal groove are so set that the flange portion of another connector connecting bolt does not come into contact with a groove surface of the seal groove of the bolt even if the other bolt substantially faces in the same direction as the bolt and interferes in a horizontal posture (HPo) or inclined posture. a head portion at or near a first axial end part; a screw portion at or near a second axial end part; a flange portion which bulges radially outwardly near and adjacent to the head portion and can come into contact with the second connector at the time of proper connection to prevent any further tightening of the bolt; at least one stepped recess which is located closer to the first axial end part than the screw portion, recessed radially inwardly and which can be fractured when an excessive tightening torque is applied; and at least one retaining groove which is located between the stepped recess and the screw portion in an axial direction and recessed radially inwardly and into which a retaining member for preventing the detachment of the bolt from the first connector by coming into contact with the first connector before connection is to be fitted, characterized by at least one seal groove which is located between the stepped recess and the flange portion in the axial direction and recessed radially inwardly and into which at least one seal member to be held in close contact with the first connector in a fluid-tight manner at the time of proper connection is to be fitted, wherein the thickness of the flange portion is larger than the width of the seal groove, wherein a width and depth of the seal groove is so set that a shaft portion leading end of another connector connecting bolt does not come into contact with a groove surface of the seal groove of the bolt even if the other bolt faces in an opposite direction relative to the bolt and interferes in an inclined posture, and wherein a thickness of the flange portion and a width and depth of the seal groove are so set that the flange portion of another connector connecting bolt does not come into contact with a groove surface of the seal groove of the bolt even if the other bolt substantially faces in the same direction as the bolt and interferes in a horizontal posture (HPo) or inclined posture.