Among each of the conductors 4a to 4e that form the flat cable 2 of the flat harness 1, at the installation part of the relay connector 6, conductors 4a and 4e, conductors 4a1 and 4a2, and conductors 4e1 and 4e2 are respectively cut and separated, and then connected to the relay connection terminal 8. Thereby, the number of conductors 4 that form the flat cable 2 is decreased to a minimum, and thus it is possible to decrease the unnecessary amount of material. In addition, in the manufacturing steps, the crimping step of the flat cable 2 to the relay connector 6, the cutting step, and the molding step are carried out in one step, and thus the number of manufacturing steps can be decreased.