A substrate module includes a first substrate and a second substrate. The first substrate comprises a base material; a transmission line in which a signal line is sandwiched between two ground patterns, on the surface of the base material; a ground pattern on the rear surface of the base material; and an exposed portion in which the ground pattern on the substrate surface is exposed by partially cutting out the base material and the ground pattern on the substrate rear surface. The second substrate comprises a base material and a transmission line in which a signal line is sandwiched between two ground patterns, on the surface of the base material. In connecting a transmission line of the first substrate and that of the second substrate, the ground patterns of the first and second substrates are fused with each other at the exposed portion and fixed.