There is provided a semiconductor device with enhanced reliability having a heat sink mounting a plurality of semiconductor chips, a plurality of inner leads connected electrically to the semiconductor chips, a molding body for resin molding the plurality of semiconductor chips and the plurality of inner leads, a plurality of wires for providing electrical connections between the respective electrodes of the semiconductor chips and the inner leads corresponding thereto, and wide outer leads connecting to the inner leads and exposed outside the molding body. A plurality of slits are formed in the respective portions of the outer leads located outside the molding body to extend lengthwise in directions in which the outer leads are extracted. This achieves a reduction in lead stress which is placed on the outer leads by thermal stress or the like after the mounting of a MOSFET and thereby enhances the reliability of the MOSFET.