A base plate in which a member to be joined is joined to one mounting surface via a joining material, wherein the joining position of the mounting surface, to which the member to be joined is joined, is provided with a concave for joining the member to be joined via the joining material. The concave is configured so that the area of the opening thereof is greater than the member to be joined, and the depression at the outer circumferential part of the concave, which faces the outer circumferential edge of the member to be joined, is deeper than the depression at the central part of the concave.