A resin-encapsulated semiconductor device includes a semiconductor element mounted on a die pad portion, a plurality of lead portions arranged so that leading end portions thereof are opposed to the die pad portion, and thin metal wires connecting electrodes of the semiconductor element to the lead portions. An encapsulation resin encapsulates the die pad portion, semiconductor element and lead portions in such a manner that a bottom surface part of the die pad portion and a lead bottom surface part, lead outer surface part, and lead upper end part of the lead portions are exposed from the encapsulation resin. A plating layer is formed on the lead bottom surface parts and the lead upper end parts. The encapsulation resin has cutouts on a side surface thereof vertically above the portions of the lead upper end parts on which the plating layer is formed.