To provide a harness cable routing structure for a thick plate material allowing easy and quick cable routing of a harness for the thick plate material having prescribed thickness in a state of the harness minimally extended and realizing of the reduction of the weight and an opening/closing device for a rear door for a vehicle including the structure.

A tapered cross section mounting guide groove 100 for burying with the whole of the diameter of the harness 120 is formed in advance on a supporting arm 41 by three-dimensional laser working. The harness 120 is cable-routed so as to be fitted into the guide groove 100 for mounting. The middle of the cable-routed harness 120 is pressed by a locking means 110.