An optical semiconductor element mounting package that has good adhesion between the resin molding and the lead electrodes and has excellent reliability is provided, as well as an optical semiconductor device using the package is also provided. The optical semiconductor element mounting package having a recessed part that serves as an optical semiconductor element mounting region, wherein the package is formed by integrating: a resin molding composed of a thermosetting light-reflecting resin composition, which forms at least the side faces of the recessed part; and at least a pair of positive and negative lead electrodes disposed opposite each other so as to form part of the bottom face of the recessed part, and there is no gap at a joint face between the resin molding and the lead electrodes.