PURPOSE: To achieve an easier assembling and a stable fixation even under a prolonged operation, by a method wherein edge sheet members whose teeth engage between individual plate pairs are joined to side sheet members to form rigid frames of a first opening part side of a heat exchange integrated body, and the four corners of the frames are joined to profile bars to form integrated frames.

CONSTITUTION: Rectangular plates 10 are joined 13 by spiral welding at an end part without any press deformed part 11 thereof to build plate pairs 14 which form a passage for a heat exchange medium leading to a first opening part 15. The first opening part 15 and the second opening part 18 of a heat exchange integrated body St are made to cross each other and to prevent the mixing of first and second fluids flowing into the first and second opening parts 15 and 18, an edge sheet member 20 is provided and tooth parts whose width (a) corresponds to the depth (t) of the press deformed part 11 of each plate pair 14 plus twice the thickness (s) of each rectangular plate 10 are formed on the sheet member 20 to engage between the individual plate pairs 14 and 14. The edge sheet members are combined with the side sheet members 30 to form rigid frames on the first opening part side and the four corners of the frames are joined to profile bars 40 to firmly hold the heat exchange integrated body St.