Abstract: A semiconductor device has a resin package containing a semiconductor chip, lead pins electrically connected to the chip, and a heat radiating plate for transmitting heat of the semiconductor chip to the exterior. The lead pins and the heat radiating plate protrude from a side wall of the resin package. The lead pins each have a planar end part attached by soldering to a wiring pattern on a circuit board. The heat radiating plate has a planar attachment part which is attached by soldering to a heat radiating pattern on the circuit board. The planar attachment part of the heat radiating plate is non-rectangular and is shaped such that when it is attached to a solder-coated area on a heat radiating pattern formed on a circuit board, the melted solder is more effectively prevented from flowing out.