An attachment part serves as a ground bar in a substantially U shape in a cross-sectional view, and has a rectangular first side wall portion provided upright along one widthwise end portion of a bottom plate portion, and an inverted L-shaped second side wall portion provided upright along the other widthwise end portion of the bottom plate portion. The first side wall portion is provided with a plurality of first slots for receiving four first signal lines exposed from first external insulators and first shield conductors of the plurality of composite cables. The second side wall portion is provided with a plurality of second slots for receiving the first shield conductors exposed from the first external insulators of the plurality of composite cables.