A seal groove is cut in the distal end face of a wrap of a fixed scroll, with a tip seal being fitted in the seal groove. Swelling members are interposed between both side faces of the tip seal and corresponding opposite side faces of the wrap. Condensed liquid of working medium w and water condensed from steam contained in the working medium w that are present between the tip seal and an end plate provide a sealing effect. When the swelling members swell upon absorbing the condensed liquids, they exert a strong pressing force on the tip seal and the wrap, thereby firmly retaining the tip seal in position. This prevents the tip seal from being pressed against the end plate, so that wear of the tip seal is reduced.