Forming aluminum circuit layers forming an aluminum circuit layers on one surface of a ceramic substrate and forming copper circuit layers are included. The copper circuit layers are formed by laminating copper boards for the circuit layers on the respective aluminum circuit layers, arranging the laminate between a pair of support boards having a convex curved surface at least on one surface so as to face to each other, moving the support boards in a facing direction to press the laminate in a lamination direction, and heating in this pressing state so that the copper boards for the circuit layers are bonded on the aluminum circuit layers respectively by solid phase diffusion. In the step of forming the copper circuit layers, the support boards are arranged so that either one of the convex curved surface is in contact with the adjacent copper boards for the circuit layers in the laminate.