A composite transmission line includes a laminated insulator including insulator layers, signal transmission lines including first and second signal transmission lines and a power transmission line. The power transmission line includes a power transmission conductor pattern along the insulator layers, and an interlayer connection conductor that interlayer-connects power transmission conductor patterns. The first signal conductor pattern of the first signal transmission line, the second signal conductor pattern of the second signal transmission line, and the power transmission conductor pattern are parallel or substantially parallel to each other on the insulator layers that are mutually different from each other. The first and second signal conductor patterns interpose a first ground conductor in the laminating direction of the insulator layers. The power transmission line is in a side portion of the first signal conductor pattern.