PROBLEM TO BE SOLVED: To enable an operation to be started after dust adhered to a filter body is blown out when an engine is started again, enable an output of the engine to be improved and operation efficiency to be improved.SOLUTION: A cooling fan [20] and a dust discharging fan [30] of which blade angles are set in opposition values are rotatably arranged on the same central axis between an engine [E] and a radiator [50]. There are provided a cooling clutch [C1] for connecting or disconnecting a transmission of driving force from the engine [E] to a cooling fan [20] and a dust discharging clutch [C2] for connecting or disconnecting a transmission of driving force from the engine [E] to the dust discharging fan [30]. There is provided a driving state changing-over means [45] to be controlled under interval state to connect the cooling clutch [C1] and the dust discharging clutch [C2] alternatively under a prescribed time interval. When the engine [E] is started to operate, the interval control is started from the connection with the dust discharging clutch [C2].SELECTED DRAWING: Figure 10
