PURPOSE: To obtain tube structural bodies inexpensively through volume production by forming convex parts and concave parts in the required places of a metal plate material having undergone solder plating or the like, bending this plate material to a U shape, and abutting and adhering said concave and convex parts, thereby forming two or more passages.

CONSTITUTION: In producing a tube structual body having two or more passages for radiators etc., a metal plate material 20 plated with a low-temp. melting adhesive agent such as solder is formed by means of forming rolls 11. Namely, shallow projecting flights 23, 24 are formed on both sides of the line extending in the central longitudinal direction of the plate material 20, and these are deep drawn to form convex parts 25, 26. Thence, a convex part 27 is formed adjacently to the convex part 25 and these are drawn to form convex part 28 and concave part 29. After an end part 30 is bent to an L shape and an end part 31 to a crank shape, the plate material is bent to a U shape to allow the concave part 26 to abut on the concave part 29, thence the material is heated in a heating furnace 32 and is thereby melt- stuck, whereby a flat tube structural body 36 having two passages 34, 35 is obtained. In this way, a large number of steps for conventional methods is eliminated.