Provided is a semiconductor device having: a terminal portion having a through hole is formed on a principal surface portion; and a casing portion in which an opening to make the principal surface portion of the terminal portion exposed is provided, wherein the opening has a corner portion corresponding to a corner of the principal surface portion of the terminal portion, wherein the casing portion has a thick portion, in which thickness of a resin may be greater than that of a middle portion between adjacent corner portions across two sides forming the corner portion, in a surrounding area of the opening. Furthermore, a slit portion extending outward from the corner portion may be formed in the casing portion. At least a part of the outline of the slit portion as viewed from the upper surface direction of the casing portion may be curved.