A transmission line device includes a first multilayer substrate with a transmission line including laminated insulating base materials and a conductor pattern on the insulating base materials, and a second multilayer substrate defining a connected member to which the transmission line of the first multilayer substrate is connected. The conductor pattern includes a signal conductor pattern and a signal electrode pad electrically connected to the signal conductor pattern. The first multilayer substrate includes a resist film provided on a surface of a laminate of the insulating base materials, and the resist film includes an opening that is separated from an outer edge of the signal electrode pad in a surface direction of the laminate of the insulating base material and exposes the signal electrode pad.