An air conditioner for a vehicle comprising: a compressor that compresses refrigerant for conditioning air in a vehicle cabin and defines a refrigeration cycle for the vehicle; a detecting section that detects a thermal load of air-conditioning in the vehicle cabin; and a control section that controls a rotational speed of the compressor such that the thermal load detected by the detecting section satisfies a predetermined air conditioning request in the vehicle cabin, wherein the detecting section is adapted to detect a post-evaporator temperature that is a temperature of air that has passed through an evaporator of the refrigeration cycle as the thermal load, and the control section is adapted to determine whether the air conditioning request in the vehicle cabin is satisfied based on a difference between the post-evaporator temperature detected by the detection section and a target post-evaporator temperature that is a target value of the post-evaporator temperature to satisfy the air conditioning request, characterized in that in the case where the predetermined air conditioning request in the vehicle cabin is satisfied by controlling the rotational speed of the compressor, the control section is configured to periodically switch between a reference rotational speed and a low rotational speed of the compressor that is lower than the reference rotational speed so as to maintain a state where the air conditioning request is satisfied, the reference rotational speed being a rotational speed of the compressor within a predetermined range that includes the rotational speed at which the air conditioning request in the vehicle cabin is satisfied such that the rotational speed is periodically controlled in order to maintain the air conditioning request. a compressor that compresses refrigerant for conditioning air in a vehicle cabin and defines a refrigeration cycle for the vehicle; a detecting section that detects a thermal load of air-conditioning in the vehicle cabin; and a control section that controls a rotational speed of the compressor such that the thermal load detected by the detecting section satisfies a predetermined air conditioning request in the vehicle cabin, wherein the detecting section is adapted to detect a post-evaporator temperature that is a temperature of air that has passed through an evaporator of the refrigeration cycle as the thermal load, and the control section is adapted to determine whether the air conditioning request in the vehicle cabin is satisfied based on a difference between the post-evaporator temperature detected by the detection section and a target post-evaporator temperature that is a target value of the post-evaporator temperature to satisfy the air conditioning request, characterized in that in the case where the predetermined air conditioning request in the vehicle cabin is satisfied by controlling the rotational speed of the compressor, the control section is configured to periodically switch between a reference rotational speed and a low rotational speed of the compressor that is lower than the reference rotational speed so as to maintain a state where the air conditioning request is satisfied, the reference rotational speed being a rotational speed of the compressor within a predetermined range that includes the rotational speed at which the air conditioning request in the vehicle cabin is satisfied such that the rotational speed is periodically controlled in order to maintain the air conditioning request.