Three bonding materials are provided on a first plate. The first bonding material is located between the second and third bonding materials. The first bonding material is thicker than the other bonding materials. A second plate is provided on the first bonding material. All of the plates and the bonding materials are heated to the softening point of the first bonding material. A load is exerted on the first bonding material to reduce the thickness of the first bonding material to that of the second and third bonding materials and transfer the load to the second and third bonding materials from the first bonding material. The temperature is raised to and kept at the crystallization point of the first bonding material. The temperature is raised to the wetting point of the second and third bonding materials.