A method of manufacturing a semiconductor device includes: preparing a bottom plate having an upper surface and a lower surface, wherein the lower surface of the bottom plate comprises a reference part and one or more inclined surfaces that are inclined with respect to the reference part, an upper portion of the one or more inclined surfaces being positioned above the reference part, and wherein a thickness of the bottom plate at the reference part is greater than a thickness of the bottom plate at the upper portion of the one or more inclined surfaces; joining a frame member to the bottom plate, at least a part of the frame member being disposed directly above the one or more inclined surfaces, a linear expansion coefficient of the frame member being smaller than a linear expansion coefficient of the bottom plate; and fixing a semiconductor element to the bottom plate.