An air conditioner for a vehicle including an engine, which serves as a power source and is configured to allow a cooling water to flow therethrough, includes a cooling water circuit, a heater, temperature sensors, and an output controller. The cooling water circuit allows the cooling water to circulate therein in a heating operation. The heater serves as a heat source, other than the engine, configured to heat the cooling water circulating in the cooling water circuit. The temperature sensors are connectable to the cooling water circuit and are configured to detect a temperature of the cooling water. The temperature sensors are positioned upstream and downstream of at least one of the engine and the heater. The output controller is configured to adjust at least an output of the heater based on the temperatures of the cooling water detected by the temperature sensors.