A motor vehicle air conditioner that is operative to cause the quantity of conditioned air to change smoothly when an air conditioner bypass passage is opened/closed. When a user switches a typical automotive air conditioner between a bi-level flow mode and a full outer-air mode, a warm-air bypass door does not stop at an intermediate position, thus causing both air quantity of conditioned air and blown-air temperature of conditioned air to change greatly. In the air conditioner of the present invention, the air conditioner warm-air bypass door stops once at an intermediate position, thus enabling the air quantity of the conditioned air and the temperature change in the blown-air to change gradually.