Disclosed therein is an air conditioner for a vehicle, which includes: a bypass passageway formed at a lower portion of an air-conditioning case which bypasses cold air and warm air passageways; a regeneration passageway formed at the lower portion of the air-conditioning case for supplying the air passing through the bypass passageway to a regeneration part of the desiccant rotor; and a heater having one side area arranged on the bypass passageway and the other side area arranged on the warm air passageway. The air conditioner heats the dehumidified air passing through the desiccant part of the desiccant rotor and the bypass passageway by the heater and supplies the heated air to the regeneration part of the desiccant rotor so as to regenerate the desiccant rotor, thereby reducing the number of components and the entire size and enhancing heating performance and increasing an air volume by moving the air, which is heated while passing the bypass passageway through a door mounted in the communication chamber, toward the warm air passageway in a heating mode.