A resin sealing semiconductor device (2) having a structure in which a portion to be sealed of components including a plurality of chip mounting board, a semiconductor chip mounted to a front surface of each chip mounting board, and a plurality of leads provided correspondingly to each chip mounting board is embedded in resin molded portions (41 and 42) molded into a generally plate shape, and outer lead portions of the plurality of leads (16 and 17) are led out in line from a side surface at one end in a width direction of the resin molded portions, and back surfaces as exposed surfaces (11u1 to 11w1 and 12u1 to 12w1) of each chip mounting board are placed on one surfaces of the resin molded portions (41 and 42), wherein a plurality of positioning protrusions (50) are provided on one surfaces of the resin molded portions (41 and 42), and a protrusion height of the positioning protrusions is set so that a gap to be filled with insulating resin is formed between each part of the exposed surface of each chip mounting board and a radiator plate when the positioning protrusions (50) are abutted against the radiator plate.