A heat exchange unit is composed of rectangular heat transfer panels with terraced flat portions, combined in parallel and integrally with each other to form first and second spaces. Outer and inner reinforcement members having respective serrations are welded to the panels so that the serrations are inserted into gaps between the heat transfer panels. The outer and inner reinforcement members are made of the same material as the heat transfer panels and have a larger thickness than the heat transfer panel, so as to bear high heat input given during the welding. The heat input caused by welding can be increased to enable portions of the heat transfer panels to be melted rapidly.