An object of the present invention is to provide a wiring harness attaching structure aiming at improvement in attaching workability and aiming at thinning of a vehicle door. Provided is a wiring harness attaching structure for attaching a wiring harness to a vehicle interior outer surface of a door trim constituting a vehicle door. The wiring harness is a flat cable whose width dimension is longer than a thickness dimension, is located inside a structure body containing gas phase, and is provided integrally with the structure body containing gas phase. The structure body containing gas phase is attached along the vehicle interior outer surface of the door trim. The wiring harness is attached to the door trim together with the structure body containing gas phase.