PURPOSE: To obtain a package for mounting semiconductor elements in which a semiconductor element can be protected against damage due to heat shock while enhancing the heat dissipation properties of the mounted semiconductor element by constituting the bottom plate of the package cavity of a substrate of sintered aluminum nitride provided with copper plate layers on the opposite sides thereof.

CONSTITUTION: In a package where a cavity 3 is formed by bonding a bottom plate 1 to one side of a frame-like circuit board 2, the bottom plate 1 is composed of a substrate 5 of sintered aluminum nitride provided with copper plate layers 4 on the opposite sides thereof.