A mixing chamber for an air-conditioning system including a cylindrical vessel having a plurality of inlet ports provided at an axial end portion for introducing cool air from an air-conditioning unit and warm air from passenger compartments. The vessel is further provided at the other axial end portion with outlet ports. The inlet and outlet ports are oriented in a tangential direction with respect to the vessel to thereby form a spiral air flow in the vessel. The vessel has a water capturing ring located along the inner wall surface of the vessel to prevent film of water on the inner wall surface of the vessel from flowing toward the outlet ports. A water drawing slit is formed in the vessel to open to the inner wall surface at a side of the water capturing ring adjacent to the inlet ports to discharge the water accumulated at this side of the ring.