PURPOSE: To manufacture a flat wire harness with excellent mechanical strength for use in electric wirings of a car, with which automation of the manufacturing process is facilitated.

CONSTITUTION: A long stretching insulative plate 20 having a plurality of holes 23 for pressure contacting is placed on a joint circuit board 11, and electric wires installed between two such joint circuit boards are led to a connector 16 via the holes 23 in pressure contacting with a circuit conductor in a case 13 of relay circuit board 11. The wire installed surface of the insulative plate 20 is covered with a protector sheet 24 having a hot melt type adhesive agent layer 27, followed by heating to cause the adhesive agent to melt, with which the spaces between the installed wires are filled so as to fix the joint circuit board 11, insulated plate 20, and protector sheet 24 with one another.