In a semiconductor device where a semiconductor element having an asymmetric temperature distribution during an operation is mounted, inner leads on the right and left ends have asymmetric lengths, so that the right and left regions of a semiconductor element mounting part have different sizes. The semiconductor element is mounted so as to have a high-temperature region side in a wide region of the mounting part, and the inner leads are wire bonded at the center to the wide region of the mounting part. It is thus possible to provide a small semiconductor device in which a long semiconductor element is mounted with heat dissipation.