A passenger compartment duct is provided with a passenger compartment air blow opening through which air is supplied into a passenger compartment and a cab-air-supply-duct communicating opening communicating with a cab air supply duct. The cab air supply duct is provided with a cab air blow opening through which air is supplied into a cab and a passenger-compartment-duct communicating opening communicating with the passenger compartment air supply duct. There are provided a damper turning communication from the passenger compartment air supply duct to the cab air supply duct ON or OFF, a cab air outlet through which air in the cab is discharged to the outside of the cab, and a damper opening or closing the cab air outlet. The cab air outlet includes a first outlet opened so as to communicate with an air intake side of cab air-conditioning units and a second outlet opened so as to communicate with the passenger compartment. The damper selectively discharges air in the cab to either the outside air intake side of the cab air-conditioning units or the passenger compartment.