PURPOSE: To enhance productivity double by simultaneously assembling two heat exchangers in an assembly line and separating the heat exchangers in a final stage.

CONSTITUTION: Side plates 1a, 2a and 1b, 2b of the respective heat exchangers 1, 2 are successively provided with each other and connecting pieces 3 provided with narrow connecting parts 3b having V-shaped cutting grooves 3a are integrally formed at both ends of the side plates 1a, 2a. The connecting pieces 3 of the assembled heat exchangers 1, 2 are cut when the connecting pieces 3 thereof are pinched by suitable tools and bending actions are repeated several times to bend these pieces to the side plates 1a, 2a. The heat exchangers 1, 2 are thereby separated to the two exchangers. The two-piece manufacture of the heat exchangers is executed in such a manner within the time required heretofore for the assembly, by which the productivity is enhanced nearly double.