A wire harness design aiding apparatus for designing a wire harness, said wire harness including a main line and at least one branch line (21, 22), and said wire harness being manufactured by using a wiring jig plate, the apparatus comprising: a display unit with a CAD display for three dimensionally displaying an object on which the wire harness is to be arranged and displaying the wire harness on the CAD display in a state that two points of the wire harness are constrained; a guide line setting unit for automatically setting guide lines which connect, in a longitudinal direction of the main line, intersection points of a virtual guide plane, which is parallel to the wiring jig plate and extends passing an axial line of the main line, and an outer surface of the main line, and displaying the guide lines on the CAD display; a branch line setting unit for branching out the branch line (21, 22) from a position on the main line and setting a branching angle between the axial line of the main line and an axial line of the branch line (21, 22); an inclination angle setting unit for setting an inclination angle between the axial line of the branch line (21, 22) and the virtual guide plane; and an inclination angle correcting unit for automatically checking as to if the inclination angle is equal to a predetermined angle or smaller, automatically correcting the inclination angle to the predetermined angle or smaller when the inclination angle exceeds the predetermined angle, and automatically correcting the branching out of the branch line so as to satisfy that the branch line is to be branched out from one of the guide lines and that the branching angle is set to the predetermined angle or smaller. a display unit with a CAD display for three dimensionally displaying an object on which the wire harness is to be arranged and displaying the wire harness on the CAD display in a state that two points of the wire harness are constrained; a guide line setting unit for automatically setting guide lines which connect, in a longitudinal direction of the main line, intersection points of a virtual guide plane, which is parallel to the wiring jig plate and extends passing an axial line of the main line, and an outer surface of the main line, and displaying the guide lines on the CAD display; a branch line setting unit for branching out the branch line (21, 22) from a position on the main line and setting a branching angle between the axial line of the main line and an axial line of the branch line (21, 22); an inclination angle setting unit for setting an inclination angle between the axial line of the branch line (21, 22) and the virtual guide plane; and an inclination angle correcting unit for automatically checking as to if the inclination angle is equal to a predetermined angle or smaller, automatically correcting the inclination angle to the predetermined angle or smaller when the inclination angle exceeds the predetermined angle, and automatically correcting the branching out of the branch line so as to satisfy that the branch line is to be branched out from one of the guide lines and that the branching angle is set to the predetermined angle or smaller.