An air conditioner, comprising: a compressor ; a first heat exchanger connected to the compressor and configured to operate as a condenser during a cooling operation and to operate as an evaporator during a heating operation; a refrigerant pipe disposed inside the first heat exchanger and configured to carry a two-phase refrigerant; a connection pipe , out of a plurality of connection pipes , connected to the refrigerant pipe and configured to guide a refrigerant separated from the two-phase refrigerant in the refrigerant pipe to a header ; the header being connected to the connection pipe ; the header being further connected to the other connection pipes out of the plurality of connection pipes ; the header being configured to guide the refrigerant flowing from the connection pipes to a bypass pipe ; a bypass pipe connected to the header and configured to guide the refrigerant to the compressor ; a flow rate control valve installed at the bypass pipe and configured to control a flow rate of the refrigerant; a subcooler configured to superheat the refrigerant flowing through the bypass pipe ; and a controller configured to control an opening degree of the flow rate control valve; characterized in that the controller is configured to control the opening degree of the flow rate control valve based on a comparison between a refrigerant flow velocity of the two-phase refrigerant inside the first heat exchanger and a reference flow velocity, wherein the controller is configured to estimate the refrigerant flow velocity of the two-phase refrigerant flowing inside the first heat exchanger in consideration of a type of the compressor , an operation frequency of the compressor , a number of paths of an outdoor unit of the air conditioner, and an outside temperature. a compressor ; a first heat exchanger connected to the compressor and configured to operate as a condenser during a cooling operation and to operate as an evaporator during a heating operation; a refrigerant pipe disposed inside the first heat exchanger and configured to carry a two-phase refrigerant; a connection pipe , out of a plurality of connection pipes , connected to the refrigerant pipe and configured to guide a refrigerant separated from the two-phase refrigerant in the refrigerant pipe to a header ; the header being connected to the connection pipe ; the header being further connected to the other connection pipes out of the plurality of connection pipes ; the header being configured to guide the refrigerant flowing from the connection pipes to a bypass pipe ; a bypass pipe connected to the header and configured to guide the refrigerant to the compressor ; a flow rate control valve installed at the bypass pipe and configured to control a flow rate of the refrigerant; a subcooler configured to superheat the refrigerant flowing through the bypass pipe ; and a controller configured to control an opening degree of the flow rate control valve; characterized in that the controller is configured to control the opening degree of the flow rate control valve based on a comparison between a refrigerant flow velocity of the two-phase refrigerant inside the first heat exchanger and a reference flow velocity, wherein the controller is configured to estimate the refrigerant flow velocity of the two-phase refrigerant flowing inside the first heat exchanger in consideration of a type of the compressor , an operation frequency of the compressor , a number of paths of an outdoor unit of the air conditioner, and an outside temperature. <EOS>