The present invention relates to an air conditioner for a vehicle, which can send air-conditioned air toward a rear seat of the vehicle in order to perform air-conditioning of a front seat of the vehicle and air-conditioning of a rear seat. The air conditioner includes an air passageway formed in an air-conditioning case, wherein the air passageway includes: a rear seat cold air passageway which is a passageway that the air passed through the heat exchanger for cooling bypasses the heat exchanger for heating and flows toward a rear seat of the vehicle; and a warm air passageway which is a passageway that the air passed through the heat exchanger for cooling passes through the heat exchanger for heating and flows toward a front seat or the rear seat of the vehicle. The air conditioner includes a rear seat temperature adjusting door for controlling an amount of air flowing from the warm air passageway to the rear seat air outlet and an amount of air flowing from the rear seat cold air passageway to the rear seat air outlet.