PURPOSE: To improve the heat radiation capacity, by providing suitable guides over and under a radiator fin and conducting run air into a radiator and causing natural convection while the vehicle is stopped.

CONSTITUTION: The guides 6, 6 are so provided over and under the radiator fin 4 so that spaces h', h are defined at the inlet and outlet ports. The forms of the guides are continuously varied toward the upper part and lower part of the radiator fin 4 almost in the center as to the direction of proceeding. Air flow caught at the inlet port is introduced into the radiator fin 4. Natural convection is caused by using the spaces h' h to cool the fin 4 while the vehicle is stopped. Thereby, the heat radiation capacity is improved