An upper surface part of an engine guard forming an engine room is formed by a bonnet for opening and closing a maintenance port and a fixed hood. On the lower surface side of the bonnet is disposed a movable duct in a state that when the bonnet is opened the movable duct is moved integrally with the bonnet so as to open the maintenance port. On the lower surface side of the fixed hood, is disposed a fixed duct in a state that when the bonnet is closed one end of the fixed duct is connected to one end of the movable duct in the air flow direction. An exhaust duct is formed by both the movable duct and the fixed duct.