A pressing area set on a main surface of a plate-shaped holding jig is arranged on contact parts. The contact parts are pressed against a multilayer board while heating the multilayer board and the pressing area of the holding jig is inclined with a warp of the multilayer board. In this way, when pressing for bonding the contact parts is performed, even if the multilayer board is warped by the heating and the contact parts are shifted, the contact parts are pressed against the multilayer board without fail.