An optical subassembly includes: a conductor plate having a pair of penetration holes, both penetrating the conductor plate from an outer surface to an inner surface; a pair of lead terminals fixed in the two respective penetration holes and passing through the pair of penetration holes; a wiring board with a pair of wiring patterns arranged on a surface; and a plurality of bonding wires electrically connecting the pair of lead terminals and the pair of wiring patterns. A cross section of the pair of lead terminals on the inner surface side is larger than a cross section on the outer surface side. End faces on the inner surface side are situated within a range from +180 μm to −100 μm to the inner side from the inner surface of the conductor plate in a direction perpendicular to the inner surface.