A convex portion projecting from a plate member includes a base, a convex body, and a guide. The base is part of the plate member. The convex body erects in a cylindrical shape from the base. The guide is at a top end of the convex body. The base includes a first inner circumferential surface, a second inner circumferential surface, and a third inner circumferential surface. The first inner circumferential surface is continuous with an inner circumferential surface of the convex body and extends in a direction opposite to an erecting direction of the convex body. The second inner circumferential surface is continuous with the first inner circumferential surface and intersects the erecting direction of the convex body. The third inner circumferential surface is continuous with the second inner circumferential surface and is larger in diameter than the first inner circumferential surface.