PURPOSE: To strengthen the bonding strength of a heat collecting plate by a structure wherein the heat collecting plate is inserted between a cylindrical rotary cooling member, in the convex goove of which a heat collecting tube is inserted, and conical rotary cooling members and the junction part of the heat collecting plate, which is jointed on the heat collecting pipe, is heated by the arc of tungsten inert gas electrode.

CONSTITUTION: A pair of conical rotary cooling members 7 and 7', which form a gap 11 above a convex groove 10 by pinchingly supporting a heat collecting plate 1 on a cylindrical rotary cooling member 6, are provided so as to joint the heat collecting plate 1 on a heat collecting pipe 3 inserted in the convex groove 10. The heat collecting plate 1 on the heat collecting pipe 3 is heated at the gap 11 by the arc of the tungsten inert gas electrode 5 equipped above the gap 11 of the cooling members 7 and 7'. Owing to the structure as described above, the arc of the electrode 5 can be concentrated only on the upper surface of the junction part of the heat collecting plate 1 and as well as the heat collecting plate 1 and the heat collecting pipe 3 are cooled with the cooling members 6, 7 and 7' and the heating and fusing heat produced with the electrode 5 is effectively transmitted to the heat collecting pipe 3, resulting in obtaining good welding.