An outdoor unit for a refrigerant circuit, the refrigerant circuit being formed by connecting, by pipes, a compressor, a condenser that causes refrigerant to reject heat to condense, a pressure reducing device that reduces a pressure of the refrigerant that is condensed, a gas-liquid separator, and an evaporator, the outdoor unit comprising: - the compressor that compresses and discharges the refrigerant sucked into the compressor; - the pressure reducing device that reduces a pressure of the refrigerant; - the gas-liquid separator that separates the refrigerant exiting the pressure reducing device into refrigerant in gaseous form and refrigerant in liquid form; - an outdoor heat exchanger serving as the evaporator including at least a plurality of heat transfer tubes that cause the refrigerant to receive heat to evaporate, and an inlet header that distributes the refrigerant entering the inlet header to the plurality of heat transfer tubes; - a gas-liquid-separator bypass pipe that causes the refrigerant in the gaseous form separated by the gas-liquid separator to bypass the evaporator; - a gas-liquid-separator-side flow control device configured to control a flow rate of the refrigerant through the gas-liquid-separator bypass pipe; - a header bypass pipe having one end connected to the inlet header at a header termination side located opposite to a refrigerant inlet side of the inlet header, the header bypass pipe sucking the refrigerant entering the inlet header; - a header-side flow control device configured to control a flow rate of the refrigerant through the header bypass pipe; and - a determination device configured to determine, from a frequency of the compressor, an opening degree of the gas-liquid-separator-side flow control device and an opening degree of the header-side flow control device. - the compressor that compresses and discharges the refrigerant sucked into the compressor; - the pressure reducing device that reduces a pressure of the refrigerant; - the gas-liquid separator that separates the refrigerant exiting the pressure reducing device into refrigerant in gaseous form and refrigerant in liquid form; - an outdoor heat exchanger serving as the evaporator including at least a plurality of heat transfer tubes that cause the refrigerant to receive heat to evaporate, and an inlet header that distributes the refrigerant entering the inlet header to the plurality of heat transfer tubes; - a gas-liquid-separator bypass pipe that causes the refrigerant in the gaseous form separated by the gas-liquid separator to bypass the evaporator; - a gas-liquid-separator-side flow control device configured to control a flow rate of the refrigerant through the gas-liquid-separator bypass pipe; - a header bypass pipe having one end connected to the inlet header at a header termination side located opposite to a refrigerant inlet side of the inlet header, the header bypass pipe sucking the refrigerant entering the inlet header; - a header-side flow control device configured to control a flow rate of the refrigerant through the header bypass pipe; and - a determination device configured to determine, from a frequency of the compressor, an opening degree of the gas-liquid-separator-side flow control device and an opening degree of the header-side flow control device.