A semiconductor device is provided with improved reliability. A semiconductor chip is mounted over a chip mounting portion of a lead frame via solder. A metal plate is arranged over a source pad of the semiconductor chip and a lead portion of a lead frame via solder. A solder reflow process is performed thereby to bond the semiconductor chip over the chip mounting portion with a solder, and to bond the metal plate to the source pad and the lead portion with the other solders. The lead frame is formed of a copper alloy, and thus has its softening temperature higher than the temperature of the solder reflow process. The metal plate is formed of oxygen-free copper, and has its softening temperature lower than the temperature of the solder reflow process, whereby the metal plate is softened in the solder reflow process. Thereafter, a gate pad electrode of the semiconductor chip is coupled to a lead portion via the wire, a sealing resin portion is formed, and then the lead frame is cut.