An insulating substrate is prepared. In this substrate, plural via holes penetrating in a thickness direction are filled with a conductive paste. This paste is produced by adding an organic solvent to a powder of an, and by processing the power of the alloy to a paste. The substrate is then pressed from a front surface and a back surface of the substrate, while being heated. The conductive paste is solid-phase sintered and interlayer connecting members are formed. A front surface protective member is disposed on a front surface of the substrate and a back surface protective member is disposed on a back surface of the substrate, and a laminate is formed. The laminate is integrated by a lower pressure being applied while heating at a lower temperature, compared to the temperature and pressure in the process of forming the interlayer connecting members.