An optical semiconductor device package includes a disc-shaped stem, metallic leads in rod form penetrating the stem in the direction of the thickness to protrude from a main surface of the stem, and a mount extending vertically from the main surface of the stem, with a plane part of the mount facing the leads. A dielectric substrate is mounted on the plane part, and an optical semiconductor chip is mounted thereon. Two impedance-adjusting dielectric substrates which are rectangular in plan view are provided extending in parallel with the leads, to cover the plane part facing the leads.