PURPOSE: To improve thermal efficiency, by a method wherein a first radiator having an inlet port tank and a second radiator having an outlet port tank are connected through an intermediate tank while an area for a heat exchanger is maximized in accordance with the configuration of a cowling.

CONSTITUTION: High-temperature cooling water flows into an inlet port tank 1 through an inlet pipe 8 and is distributed into a plularity of tubes 4 while heat exchange between the cooling water and ambient air is effected during passing through the tubes whereby the cooling water is cooled. The cooling water in the tubes 4 at the central side passes through a first radiator 100 and, thereafter, flows into an outlet port tank 2 through a second radiator 110 directly. On the other hand, the cooling water in the tubes 4 at the side of both ends flows once into an interdediate tank 3 and, thereafter, flows into the outlet port tank 2 through connecting tubes 6. According to this method, the configuration of the radiators may be designed so as to have configurations in accordance with the widthwise configuration of the cowling and the heat exchanging section of the radiator may be widened to maximum, therefore, heat exchanging efficiency may be improved.