A semiconductor element housing package includes a rectangular ceramic package having a recess section on an upper surface thereof or a penetration section from the upper surface to a lower surface thereof, and a heat radiation plate attached to the lower surface of the ceramic package, extending from one side toward the other side of the lower surface up to a region in which the heat radiation plate overlays the recess section or the penetration section, which plate has a width on a side of the other side which is narrower than that on a side of one side. The package includes a plurality of first lead pins disposed on the lower surface of the ceramic package along the other side, and a pair of second lead pins disposed on the lower surface of the ceramic package on both sides of a narrow portion of the heat radiation plate.