A back metal layer (16, 31) has a plurality of stress relaxation spaces (17). Each stress relaxation space (17) is formed to open at least at one of the front surface and the back surface of the back metal layer (16, 31). A region in the back metal layer (16, 31) that is directly below a semiconductor device (12) is defined as a directly-below region (A1), and a region outside the directly-below region (A1) that corresponds to and has the same dimensions as the directly-below region (A1) is defined as a comparison region (A21). The volume of the stress relaxation spaces (17) in the range of the directly-below region (A1) is less than the volume of the stress relaxation spaces (17) formed in the range of the comparison region (A21).