A resin multilayer substrate includes a substrate main body including first, second, and third wiring portions connected to one another by a connecting portion. First, second and third external connection terminals are respectively included in the first, second and third wiring portions. The first external connection terminal includes a conductor exposed at a surface of the substrate main body. The second and third external connection terminals include connectors mounted on conductors on the surface of the substrate main body. An auxiliary mounting conductor is disposed between the first external connection terminal and the second and third external connection terminals on the surface of the substrate main body.