A route information inputting section inputs route information including route drawing information on routes of a plurality of wiring harnesses to be arranged within a vehicle and route length information of each wiring harness. A wiring information memory stores wiring information including auxiliary unit information on a plurality of auxiliary units connected to the plurality of wiring harnesses and wire information on attributes of a plurality of wires connected between the plurality of auxiliary units. A display section displays on a screen a route drawing based on the route information input. An auxiliary unit position assigning section assigns positions of the plurality of auxiliary units on the route drawing displayed on the screen based on the auxiliary unit information included in the wiring information stored by the wiring information memory. A route/wiring linking section mutually wire-connects the plurality of auxiliary units of which positions have been assigned by the auxiliary unit position assigning section through a selective route on the route drawing, and links the wiring information on the plurality of wire-connected auxiliary units to the route information on the selected route.