First cooling air (50) enters an engine room (1) through a cooling air inlet port (7a) from the exterior of the engine room (1), and is throttled by a suction tube (8a) after passing heat exchangers such as an intercooler (6a), an oil cooler (6b) and a radiator (6c), followed by entering a centrifugal fan (4). The first cooling air (50) is then blown off toward an outer circumference of the centrifugal fan (4). Second cooling air enters the engine room (1) through cooling air inlet ports (7b, 7b) from the exterior of the engine room (1), and flows around an engine (5), various electric equipments such as an alternator (10), and an oil pan (14) while cooling them. The second cooling air is then throttled by a suction tube (8b) before entering the centrifugal fan (4), and blown off toward the outer circumference of the centrifugal fan (4). Two streams of the first and second cooling air (50, 51) are discharged to the exterior through an exhaust port (9). This arrangement enhances the degree of sealing on the engine (5) side to reduce noise, and improves the effect of cooling the electric equipments (10) for higher reliability of the electric equipments (10).