A method of mechanically producing in a continuous process a cross-fin type heat exchanger having a large fin pitch for use as a natural convection type radiator or a solar heat collector including heat collecting surfaces formed by bending the fins. A plurality of openings, respectively, accommodating a pipe therein, are formed in a continuous plate and the plate is cut into a plurality of strips of a width equal to a predetermined fin width. The strips are twisted through 90.degree. while being fed and set on a jig in such a manner that the openings formed in the strips are aligned in a straight line and the strips are spaced apart from one another by a predetermined fin pitch. The pipes are inserted in the openings in the strips in such a manner that each pipe extends through the openings in the adjacent strips set on the jig. The pipes are secured to the strips so that the strips serve as fins, and the strips are cut at any time as desired after being set on the jig.