A pressure-contact semiconductor device (100) includes thermal buffer plates (2) and main electrode blocks (3) having flanges (4), by which semiconductor substrate (1) having a pair of electrodes is sandwiched, disposed opposed to each side thereof, wherein the semiconductor substrate (1) is sealed in a gastight space by joining the flanges (4) to insulating container (5). The semiconductor device (100) is configured such that the outermost periphery of the semiconductor substrate (1) is enclosed by hollow cylindrical insulator (9) fitted on outer peripheries of the main electrode blocks (3) in the gastight space with O-rings (8) fitted between the main electrode blocks (3) and the cylindrical insulator (9), and sealed with reaction force from the O-rings (8).