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.