An air-conditioning system for a passenger car of transportation comprising an air conditioning duct configured to extend in a passenger car longitudinal direction at a ceiling portion of the passenger car and supply conditioned air discharged from an air conditioning apparatus into an inside of the passenger car, the air-conditioning system further comprising: a main duct configured to receive the conditioned air from the air-conditioning apparatus and configured to be positioned in a substantially center portion in a passenger car width direction; a chamber duct configured to discharge the conditioned air into the inside of the passenger car and configured to be positioned to be adjacent to the main duct in the passenger car width direction; and a branch duct configured to discharge the conditioned air into the inside of the passenger car and configured to be positioned at a passenger car end with respect to the chamber duct in the passenger car longitudinal direction, characterized in that the air-conditioning apparatus has a first air-conditioning apparatus and a second air-conditioning apparatus, the main duct having a partitioning plate which is configured to extend in the passenger car longitudinal direction and is configured to extend to connect a set of diagonal positions at both ends of the main duct in the passenger car longitudinal direction, so that the partitioning plate divides the main duct into a first chamber and a second chamber, the branch duct having a first rear branch duct which is configured to receive the conditioned air from the first air-conditioning apparatus through the first chamber, and a second front branch duct which is configured to receive the conditioned air from the second air-conditioning apparatus through the second chamber, the first rear branch duct and the second front branch duct being configured to be arranged in the passenger car width direction at one passenger car end, a discharge volume of the conditioned air discharged from the second front branch duct into the inside of the passenger car being higher than a discharge volume of the conditioned air discharged from the first rear branch duct into the inside of the passenger car, a discharge volume of the conditioned air discharged from the chamber duct into the inside of the passenger car being higher than a discharge volume of the conditioned air discharged from the branch duct into the inside of the passenger car. a main duct configured to receive the conditioned air from the air-conditioning apparatus and configured to be positioned in a substantially center portion in a passenger car width direction; a chamber duct configured to discharge the conditioned air into the inside of the passenger car and configured to be positioned to be adjacent to the main duct in the passenger car width direction; and a branch duct configured to discharge the conditioned air into the inside of the passenger car and configured to be positioned at a passenger car end with respect to the chamber duct in the passenger car longitudinal direction, characterized in that the air-conditioning apparatus has a first air-conditioning apparatus and a second air-conditioning apparatus, the main duct having a partitioning plate which is configured to extend in the passenger car longitudinal direction and is configured to extend to connect a set of diagonal positions at both ends of the main duct in the passenger car longitudinal direction, so that the partitioning plate divides the main duct into a first chamber and a second chamber, the branch duct having a first rear branch duct which is configured to receive the conditioned air from the first air-conditioning apparatus through the first chamber, and a second front branch duct which is configured to receive the conditioned air from the second air-conditioning apparatus through the second chamber, the first rear branch duct and the second front branch duct being configured to be arranged in the passenger car width direction at one passenger car end, a discharge volume of the conditioned air discharged from the second front branch duct into the inside of the passenger car being higher than a discharge volume of the conditioned air discharged from the first rear branch duct into the inside of the passenger car, a discharge volume of the conditioned air discharged from the chamber duct into the inside of the passenger car being higher than a discharge volume of the conditioned air discharged from the branch duct into the inside of the passenger car.