A vehicle air conditioning system secures an appropriate temperature difference in the air blown through vertically arranged air ducts when the system is in a bi-level mode. A hot air inlet is disposed above a cool air inlet. A face opening is disposed at an upper location while foot openings are disposed at a lower location. A blow mode-changing door for opening and closing both the openings is a butterfly type door. The blow mode-changing door is pivotally disposed in a door housing. In the bi-level mode in which the blow mode-changing door opens both the foot openings and the face opening, a bi-level mode bypass passageway for introducing cool air from the cool air inlet into the face opening is defined between an extremity of the blow mode-changing door and the inner wall surface of the door-housing portion.