An air conditioner for a vehicle is provided. An internal air passage in a case of the air conditioner includes a first passage which guides a portion of the air from a blower unit to flow through an auxiliary cooling portion of the evaporator, a second passage, which guides the remaining portion of the air from the blower unit to bypass the auxiliary cooling portion of the evaporator, an air mixing portion in which the air passed through the first passage and the air passed through the second passage flow together, and a third passage, which is connected to the air mixing portion and guides the air passed through the air mixing portion to flow through a main cooling portion of the evaporator. In the air conditioner a portion of the air that flows in is cooled twice while passing through the auxiliary cooling portion and the main cooling portion of the evaporator, so that a higher cooling efficiency requirement, especially regarding the early stage cooling performance, is satisfied. In addition, the air conditioner can be made slim.