A heat exchanger includes a first part made of metallic core material and having a first connecting portion, and a second part made of metallic core material and having a second connecting portion for connection to the first connecting portion. At least one of the first part and second parts is covered with a brazing layer, and the first and second parts are connected to one another by melting the brazing layer after the first part and the second part are assembled. The first connecting portion includes a fitting concave portion, and the second connecting portion includes a fitting convex portion which is press fitted into the fitting concave portion. The first connecting portion and the second connecting portion have the metallic core material exposed to each surface thereof so that each metallic core material directly contacts with one another when the convex portion is fitted in the concave portion. In this way, it is possible to fix the first part with the second part temporarily with a simple method and without more time and expense than needed.