PURPOSE: To provide a cable line manufacturing method by which a cable line having a liquid passage can be manufactured by housing cables in a conduit at the time of manufacturing the cable line.

CONSTITUTION: While a power or communication cable 3 is made to run through a die 13, a clearance is provided around the cable 3 so that a liquid passage can be formed and a plastic pipe 1 is extruded from the die 13 by applying an internal pressure. Then recesses are formed on the external surface of the pipe 1 along the length direction of the pipe 1 at facing positions while the pipe 1 is in a moldable state in an outside mandrel 14 after the pipe 1 is extruded. Thereafter, new recesses are formed on the internal surface of the corresponding pipe 1 by making the recesses formed on the external-surface of the extruded pipe 1 smooth by pressing the recesses against the internal surface of the mandrel 14 in the mandrel 14 by utilizing the applied internal pressure. Thus thin parts are formed on the plastic pipe 1 at positions facing to each other.