A method for manufacturing a semiconductor device comprising: - a step of mounting a semiconductor element to a first side of a lead frame, the lead frame extending in a lateral direction (x); - a first transfer molding step of enclosing the semiconductor element in a first insulation resin portion that is in direct contact with the first side of the lead frame; wherein the first insulation resin portion has a first skirt portion protruding downward of the lead frame from an outer peripheral portion thereof to first and second corner portions (R1, R2); - a second transfer molding step of forming a second insulation resin portion on a second, heat dissipation, side of the lead frame which is opposite to the first side; and - a step of fixing a first, upper, surface of a heatsink to a heat dissipation surface (8b2) of the second insulation resin portion, wherein the second insulation resin portion has a thin molded portion having a thickness of 0.02 mm to 0.3 mm and having a first surface being in direct contact with the second side of the lead frame and a second surface being in contact with the first surface of the heatsink, and a second skirt portion formed at an end of the thin molded portion, the second skirt portion comprising a first protrusion extending downward along a side surface of the heatsink from the thin molded portion to the first corner portion and a second protrusion extending from the first protrusion in the lateral direction (x) towards the periphery of the semiconductor device, wherein the first skirt portion covers the second skirt portion, an outer peripheral surface of the first protrusion extends from a first end in contact with the lead frame and the first skirt portion to a second end in contact with the first corner portion and forms a first interface portion between the first skirt portion and the second skirt portion in a direction perpendicular to the lateral direction (x); and wherein an outer peripheral surface part of the second protrusion extends in the lateral direction (x) from the first corner portion to the second corner portion has a flat plane shape and forms a second interface portion between the first skirt portion and the second skirt portion (8a, 82). - a step of mounting a semiconductor element to a first side of a lead frame, the lead frame extending in a lateral direction (x); - a first transfer molding step of enclosing the semiconductor element in a first insulation resin portion that is in direct contact with the first side of the lead frame; wherein the first insulation resin portion has a first skirt portion protruding downward of the lead frame from an outer peripheral portion thereof to first and second corner portions (R1, R2); - a second transfer molding step of forming a second insulation resin portion on a second, heat dissipation, side of the lead frame which is opposite to the first side; and - a step of fixing a first, upper, surface of a heatsink to a heat dissipation surface (8b2) of the second insulation resin portion, the second insulation resin portion has a thin molded portion having a thickness of 0.02 mm to 0.3 mm and having a first surface being in direct contact with the second side of the lead frame and a second surface being in contact with the first surface of the heatsink, and a second skirt portion formed at an end of the thin molded portion, the second skirt portion comprising a first protrusion extending downward along a side surface of the heatsink from the thin molded portion to the first corner portion and a second protrusion extending from the first protrusion in the lateral direction (x) towards the periphery of the semiconductor device, wherein the first skirt portion covers the second skirt portion, an outer peripheral surface of the first protrusion extends from a first end in contact with the lead frame and the first skirt portion to a second end in contact with the first corner portion and forms a first interface portion between the first skirt portion and the second skirt portion in a direction perpendicular to the lateral direction (x); and wherein an outer peripheral surface part of the second protrusion extends in the lateral direction (x) from the first corner portion to the second corner portion has a flat plane shape and forms a second interface portion between the first skirt portion and the second skirt portion (8a, 82).