Abstract: A plastic-encapsulated semiconductor device is provided, which is capable of efficient heat dissipation without upsizing while preventing the moisture from reaching an IC chip. This device is comprised of an electrically-conductive island having a chip-mounting area, an IC chip fixed on the chip-mounting area of the island, leads electrically connected to bonding pads of the chip through bonding wires, a plastic package for encapsulating the island, the chip, the bonding wires, and inner parts of the leads. The package has an approximately flat bottom face. Outer parts of the leads are protruded from the package and are located in approximately a same plane as the bottom face of the package. The island has an exposition part exposed from the package at a location excluding the chip-mounting area. A lower face of the exposition part of the island is located in approximately a same plane as the bottom face of the package. The chip and the chip-mounting area of the island are entirely buried in the package.