Provided is a package for housing an optical semiconductor element including: a metal-made base body; a metal-made lid body; a light-transmitting member; and an input/output terminal joined to the upper principal surface of the base body so as to cover a through bore. The input/output terminal includes: a dielectric-made first plate portion; a line conductor extending from one base body-side edge to another edge opposite thereto; a dielectric-made second plate portion stacked with its one base body-side end face made flush with the first plate portion; a metal plate made higher than the first plate portion and stacked with its one base body-side end face made flush with the first plate portion; and a lead terminal attached to the lower surface of the input/output terminal lying inside the through bore and electrically connected to the line conductor.