An outer shell structure surrounds a heat exchange unit formed of plates, which has first to third pairs of opposite planes. The structure has opposite end walls, opposite first walls and opposite second walls, and gap closure members. The end walls are placed on the pair of first opposite planes of the heat exchange unit. The first walls having openings are placed outside apart from the pair of second opposite planes of the heat exchange unit and connected water-tightly to edges of the end walls. The second walls having openings are placed outside apart from the pair of third opposite planes of the heat exchange unit and connected water-tightly to edges of the end walls. The gap closure members close a gap between the first walls and the heat exchange unit and a gap between the second walls and the heat exchange unit.