An electrical or optical connector system having a first connector being attached to a casing and a second connector to be connected to the first connector, wherein the first connector is comprising: a flange projecting from the outer circumference face of a cylindrical body, wherein the cylindrical body is configured to receive a cylindrical body of the second connector in an axial direction thereof, the flange includes a depressed face section configured to accommodate a mating flange of the second connector, characterized in that the mating flange is projecting outwardly from the outer circumference of an outer cylindrical body of the second connector, the mating flange further includes a protruding edge protruding toward the flange at an edge thereof for covering a corresponding depressed face section of the flange, wherein the thickness of the mating flange is set equal to the protruding length of a protruding face section from the depressed face section and the mating flange has a protruding edge that forms a generally even flat face with the outer circumferential face of the protruding face section at the inner circumferential face, by contacting with the outer circumferential face of each depressed face section, wherein said flange of the first connector further includes a screw through hole for inserting a screw to attach the flange to the casing, wherein said mating flange of the second connector further includes a screw through hole for inserting a screw into the screw hole corresponding to the mating screw through holes of the first connector configured to put a screw into the screw through hole of the second connector and the screw hole of the first connector so as to screw into the casing. a flange projecting from the outer circumference face of a cylindrical body, wherein the cylindrical body is configured to receive a cylindrical body of the second connector in an axial direction thereof, the flange includes a depressed face section configured to accommodate a mating flange of the second connector, characterized in that the mating flange is projecting outwardly from the outer circumference of an outer cylindrical body of the second connector, the mating flange further includes a protruding edge protruding toward the flange at an edge thereof for covering a corresponding depressed face section of the flange, wherein the thickness of the mating flange is set equal to the protruding length of a protruding face section from the depressed face section and the mating flange has a protruding edge that forms a generally even flat face with the outer circumferential face of the protruding face section at the inner circumferential face, by contacting with the outer circumferential face of each depressed face section, wherein said flange of the first connector further includes a screw through hole for inserting a screw to attach the flange to the casing, wherein said mating flange of the second connector further includes a screw through hole for inserting a screw into the screw hole corresponding to the mating screw through holes of the first connector configured to put a screw into the screw through hole of the second connector and the screw hole of the first connector so as to screw into the casing.