Heat exchanger with the above fluid channel

A heat exchange plate member (12) has a pair of heat exchange plates (12a, 12b) on each of which projecting ribs (14, 140) and a fluid channel forming portions (15) which are arranged alternately. The plates of the pair are connected to each other such that the projecting ribs that are formed in the plates, facing outwardly, and the projecting ribs are formed in one of the plates, are connected to a fluid channel forming regions, which in the other plates are connected at a temperature at which the strength of the material is not reduced at a joining process.