A mechanism for fixing a transmission cable enables a wiring shape of a flat cable to be held relatively reliably when the cable is acted upon by torsion force, tensile force and the like from the outside without the cable skin sustaining damage. A foldable insulation sheet is joined in folding surfaces, and a position of the flat cable is regulated by at least the fold portion thereof and joint portion. The mechanism is includes a flat cable, a foldable insulation sheet, and joint portions for joining folding surfaces of the insulation sheet. At least one of the fold portion of the insulation sheet and the joint portions is disposed in a position for regulating the flat cable. Thus, the flat cable is nipped by the folding surfaces of the insulation sheet, and its wiring shape is held by the fold portion or the joint portion.