PURPOSE: To perform two-stage type cooling and to reduce the size of the whole of a cooler by a method wherein an oil cooler and an after-cooler are vertically combined together, 8 partition plate is disposed therebetween, and cooling air is fed from a lower part to an upper part.

CONSTITUTION: An after-cooler 1 is disposed to a lower part and an oil cooler 2 to an upper part, and partition plates 3 and 4 by which the coolers are partitioned into a plurality of rooms are provided. Cooling air A is sucked through an opening part 8 with the aid of a cooling fan and simultaneously cooling air C is sucked from the motor side. Thereafter, the after-cooler 1 is cooled by means of the cooling air A and the cooling air C is caused to flow through the motor side and a partition plate 7. After the oil cooler 2 is cooled by means of cooling air B generated by joining together the cooling air A and C', the cooling air is exhausted through an exhaust duct 5 to produce exhaust gas D. This constitution performs 2-stage type cooling and reduces the size of the whole of a cooler.