A cabin system to be mounted on a vehicle body includes a driver's seat, an air conditioner body for indoor air conditioning disposed downwardly of the driver's seat, a plurality of support pillars constituting a cabin framework, a ceiling portion, and a main duct having a hollow tubular construction, the main duct forming therein an air passageway for the indoor air conditioning. The main duct includes, at a lower portion thereof, a duct connecting opening to be connected to the air conditioner body and also includes, at a lateral portion thereof, a support pillar engaging face. The main duct is attached through the support pillar engaging face to one of the support pillars along an indoor side peripheral face of the one support pillar. The support pillar engaging face of the main duct and the indoor side peripheral face of the one support pillar have profiles complementary with each other.