According to one embodiment, an optical transmission line holding member includes a holding member body, a plurality of holding holes, a plurality of electrical interconnections, and a plurality of grooves. The holding member body includes an optical semiconductor element mounting surface and an opposite surface thereof and configured to hold optical transmission lines. The holding holes are formed to penetrate between the optical semiconductor element mounting surface of the holding member body and the opposite surface thereof, the holding holes having an opening on the optical semiconductor element mounting surface side. The electrical interconnections are provided on a part of the optical semiconductor element mounting surface and electrically connected to the optical semiconductor element. The grooves are provided adjacent to the openings of the holding holes in a part of a region of the optical semiconductor element mounting surface except a region in which the electrical interconnections.