PURPOSE: To improve the bonding strength between an aluminum nitride substrate and a copper plate by forming the thin film of an oxide selected from Al2O3, Y2O3 and Sin. on the surface of the aluminum nitride substrate, superposing a copper plate on the formed thin film, and subsequently heating at a prescribed temperature.

CONSTITUTION: The thin film (having a thickness of 0.3-3.0μm) 2 of an oxide selected from Al2O3, Y2O3 and SiO2 is formed on the surface of an aluminum nitride substrate 1 preliminarily boiling-washed. The thin film 2 is subjected to an oxidation treatment at approximately 300°C for 10min in air, and brought into contact with a copper plate 3 having a copper oxide layer 4 formed on a surface thereof, and heated at a temperature of from the eutectic temperature between both the copper and the copper oxide to 1068-1075°C which are below the melting point of the copper to form an eutectic layer 5 between the substrate 1 and the copper plate 3, thereby both the substrate 1 and the copper plate 3 are strongly combined with each other.