An encapsulated semiconductor device includes: a first conduction path formative plate (1); a second conduction path formative plate (5) joined to the first conduction path formative plate; a power element (12) bonded to the first conduction path formative plate; a heatsink (14) held by the first conduction path formative plate with an insulation sheet (13) interposed between the heatsink and the first conduction path formative plate; and an encapsulation resin (9) configured to encapsulate the first and second conduction path formative plates. A through hole (3) or a lead gap (1b) is formed in a region of the first conduction path formative plate in contact with the insulation sheet. The insulation sheet is press-fitted into the through hole or the lead gap.