An air conditioner for vehicles includes a damper for switching among an outside-air inlet mode, an inside-air inlet mode and an outside/inside-air inlet mode, a manual switch for selecting any one of the respective inlet modes, an automatic switch for automatically selecting one of the respective inlet modes, a heating and cooling operation unit, an inside-air temperature sensor, an outside-air temperature sensor and a control unit for selectively controlling the respective inlet modes based on the condition of the heating and cooling operation unit, the condition of the respective switches and the difference between the temperatures detected by the respective temperature sensors. In the air conditioner, an optimum air conditioning in correspondence to the load required for air conditioning can be realized from the viewpoint of saving of energy and efficient operation.