Abstract: A heat exchanger adapted for use as a hot water radiator for a car heater, or the like, in which a plurality of tubes and a plurality of fins secured thereon extend between two fluid tanks. The fluid tanks each comprise an inner and an outer tank element, both made of synthetic resin. The inner tank element is joined via gaskets with the end portions of the tubes extending therethrough, by means of tube-expansion working, while the outer tank element is united with the inner tank element along corresponding opposed peripheral edges thereof arranged in abutting relation. A pair of partition walls are provided in opposed relation between the inner and outer tank elements, which are joined together in an abutting manner directly or via part of the gasket, to provide a fluid tight partition between an inlet chamber and an outlet chamber defined by the partition walls and the tank elements.