The invention provides an air conditioner that is capable of performing optimal control in accordance with the environment of an indoor space at startup, and a method of controlling such. The air conditioner forms a refrigerant circuit having a first heat exchanger, a second heat exchanger, thermistors, humidity sensors, a temperature sensor, ventilation fans, a compressor, a casing, a control unit, and the like. The control unit is connected to the temperature sensor, the humidity sensors, a storage unit, a timer, a manual input unit, air passageway switching mechanisms, a four-way switching valve, and an expansion valve. In the stage before the air conditioner starts normal operation after startup, the control unit performs a priority control operation that prioritizes either the sensible heat process or the latent heat process.