PURPOSE: To simplify the identification mark forming work of a flat cable by adding colored strand or strands to one or both sides of a core wire to then flatten the cable, and cutting to remove the sheath side edge and colored strand while retaining the color transferred portion of the cable.

CONSTITUTION: Colored strand or strands 3 are added in parallel with one or both sides of core wires 1-1 to 1-n arranged in paralle, and are then flattened. Then, the side edge 200 of the sheath 2 and colored strand 3 of the cable are cut to be removed therefrom while retaining the portion 5 of the sheath 2 color transferred from the strand 3. Using, for example, colored strand 3 coated with coloring agent on the outer periphery of a piando wire, the coloring agent is transferred to the sheath 2 side by means of flattening treatment such as heating or the like. Then, the side edge 200 of the cable is cut using a slit knife 4. Thus, the core wire 1 itself is colord or the side of the sheath 2 is colored by the coloring agent in a step in the conventional process, which is eliminated so as to simplify the formation of the identification mark of the cable