PURPOSE: To provide a resin sealed semiconductor device with a radiation fin wherein a package warps little.

CONSTITUTION: The title device is constituted of a metallic radiation fin 3 with an island region 2 for mounting a semiconductor chip 1, the semiconductor chip 1 fixed to the island region 2 by a conductive material such as solder, a lead 4 arranged in a periphery of the island region 2 and a resin sealing part 5 covering a mounting surface of the semiconductor chip 1 in the island region 2 and a part of the lead 4. A rear of a mounting surface of the semiconductor chip 1 in the island region 2 is exposed to an outside and is flush with the resin sealing part 5. A slit 9 is provided in the radiation film 3 along the resin sealing part 5 near the resin sealing part 5. Since the slit 9 is provided, an adhesion area between the resin sealing part 5 and the radiation film 3 is reduced and bending stress applied to the radiation film 3 is restrained.