An object of the present invention is to provide a specific relationship to be achieved by numerical values selected to sustain a desired level of coolant distributability as well as miniaturization and weight reduction for tanks in a heat exchanger adopting a structure in which the width of tubes therein is set smaller relative to the inner diameter of the tanks. In the heat exchanger according to the present invention, the inner diameter of the tanks is set small relative to the tube width, and with Dt representing an equivalent diameter at the passage section of the tanks and L representing the length of the longest path extending from an entrance to an open end of a tube, 15.ltoreq.L/Dt.ltoreq.42 is true.