Wire harness design supporting method and program

To easily transform three-dimensional data of a wire harness into a two-dimensional drawing. Wire harness data of a wire harness to be routed in a desired object is inputted to a computer. The wire harness data includes coordinate information which is three-dimensionally designed in a state adapted to the object of routing. A development plane for two-dimensionally developing the wire harness data is set. The wire harness data is divided into a plurality of segments S1 to S5 at predetermined division points, and is sequentially developed to the development plane on the segment unit basis from a start point P1 of a reference trunk line 17 via a branch line 18 to a termination point, thereby two-dimensionally developing the wire harness data on the segment unit basis so that all of the segments S1 to S5 are included in the development plane.