A package for an optical semiconductor element is provided. The package includes: a stem body having a sealing hole therein; and a lead pin having a glass sealing portion which is sealed with sealing glass in the sealing hole. Characteristic impedance of the glass sealing portion is adjusted to a given value. The characteristic impedance Zo is given by: Zo=(138/Er1/2)×log(D/d), where a hole diameter of the sealing hole is D, a wire diameter of the lead pint is d, and a dielectric constant of the sealing glass is Er, and the dielectric constant Er of the sealing glass is set by controlling an amount of bubble contained in the sealing glass.