To obtain a joining part structure having excellent wettability, without generating a continuous brittle phase, capable of preventing cracks and having sufficient toughness by adding a specified quantity of metal powder selected among Ni, Cr, Ni-Cr alloy, SUS.

Metal powder of, by weight, ≥1% to <10% selected among Ni, Cr, Ni-Cr alloy, SUS is added into a powdery brazing filler nickel. Added powder of ≤20 μm in grain size is used. This is the reason why the grain size of ≤ a size of a gap is preferable in order to simultaneously make added powder flow into the gap when molten brazing filler Ni enters the gap between interposing allowances of a plate stock by capillary tube phenomina. When tubes 11 through which coolant is made to pass are attached to an end plate, the brazing filler is coated onto root parts of the tubes 13 to the end plate 13, the whole is heated within a vacuum furnace and the brazing filler nickel is made to flow into the gaps of the tubes 11.