PROBLEM TO BE SOLVED: To reduce the cost of a submarine cable by a satisfactory connection of cables having different conductor section areas.SOLUTION: When power cables 10, 20 having different conductor section areas of central conductors 10a, 20a are mutually connected, and when the mutual central conductors 10a, 20a formed of a plurality of conductor element wires 1, 2 which mutually have different element wire diameters and are concentrically twisted with the same number of layers are connected, the conductor element wires 1 constituting the one central conductor 10a are connected to the conductor element wires 2 constituting the other central conductor 20a, by welding at each concentrically twisted layer. By repeating the above process, a cable connection portion 101, in which the entire conductor element wires are welded, is formed. With this, a submarine cable 100 having satisfactory connection between the power cable 10 and the power cable 20, via the cable connection portion 101 of a flexible joint, is manufactured.