An air conditioner comprising: a compressor (201, 301, 901) configured to compress refrigerant; a condenser (205, 305, 905) configured to receive the refrigerant compressed in the compressor (201, 301, 901); an evaporator (209, 309, 909) configured to evaporate the refrigerant passing through the condenser (205, 305, 905), the evaporator (209, 309, 909) comprising a refrigerant pipe (411, 431, 451); and a gas-liquid separator comprising: a connection pipe connected to the refrigerant pipe (411, 431, 451) of the evaporator (209, 309, 909), the refrigerant pipe being configured to carry a two-phase refrigerant; a header connected to the connection pipe and configured to receive a gas-phase refrigerant separated from the two-phase refrigerant in the refrigerant pipe; a bypass pipe connected to the header and configured to guide the gas-phase refrigerant to the compressor (201, 301, 901); a flow rate control valve installed at the bypass pipe; and a controller configured to selectively open and close the flow rate control valve based on a preset condition, wherein the two-phase refrigerant comprises a liquid-phase refrigerant and the gas-phase refrigerant, and wherein the controller is configured to: estimate a first refrigerant flow velocity and a second refrigerant flow velocity of the two-phase refrigerant based on at least one of a type of the compressor (201, 301, 901), an operation frequency of the compressor (201, 301, 901), a number of paths of the evaporator (209, 309, 909), or an outside temperature; and control flow of the gas-phase refrigerant through the bypass pipe based on a comparison between the first refrigerant flow velocity and the second refrigerant flow velocity. a compressor (201, 301, 901) configured to compress refrigerant; a condenser (205, 305, 905) configured to receive the refrigerant compressed in the compressor (201, 301, 901); an evaporator (209, 309, 909) configured to evaporate the refrigerant passing through the condenser (205, 305, 905), the evaporator (209, 309, 909) comprising a refrigerant pipe (411, 431, 451); and a gas-liquid separator comprising: a connection pipe connected to the refrigerant pipe (411, 431, 451) of the evaporator (209, 309, 909), the refrigerant pipe being configured to carry a two-phase refrigerant; a header connected to the connection pipe and configured to receive a gas-phase refrigerant separated from the two-phase refrigerant in the refrigerant pipe; a bypass pipe connected to the header and configured to guide the gas-phase refrigerant to the compressor (201, 301, 901); a flow rate control valve installed at the bypass pipe; and a controller configured to selectively open and close the flow rate control valve based on a preset condition, wherein the two-phase refrigerant comprises a liquid-phase refrigerant and the gas-phase refrigerant, and wherein the controller is configured to: estimate a first refrigerant flow velocity and a second refrigerant flow velocity of the two-phase refrigerant based on at least one of a type of the compressor (201, 301, 901), an operation frequency of the compressor (201, 301, 901), a number of paths of the evaporator (209, 309, 909), or an outside temperature; and control flow of the gas-phase refrigerant through the bypass pipe based on a comparison between the first refrigerant flow velocity and the second refrigerant flow velocity. a connection pipe connected to the refrigerant pipe (411, 431, 451) of the evaporator (209, 309, 909), the refrigerant pipe being configured to carry a two-phase refrigerant; a header connected to the connection pipe and configured to receive a gas-phase refrigerant separated from the two-phase refrigerant in the refrigerant pipe; a bypass pipe connected to the header and configured to guide the gas-phase refrigerant to the compressor (201, 301, 901); a flow rate control valve installed at the bypass pipe; and a controller configured to selectively open and close the flow rate control valve based on a preset condition, wherein the two-phase refrigerant comprises a liquid-phase refrigerant and the gas-phase refrigerant, and wherein the controller is configured to: estimate a first refrigerant flow velocity and a second refrigerant flow velocity of the two-phase refrigerant based on at least one of a type of the compressor (201, 301, 901), an operation frequency of the compressor (201, 301, 901), a number of paths of the evaporator (209, 309, 909), or an outside temperature; and control flow of the gas-phase refrigerant through the bypass pipe based on a comparison between the first refrigerant flow velocity and the second refrigerant flow velocity. estimate a first refrigerant flow velocity and a second refrigerant flow velocity of the two-phase refrigerant based on at least one of a type of the compressor (201, 301, 901), an operation frequency of the compressor (201, 301, 901), a number of paths of the evaporator (209, 309, 909), or an outside temperature; and control flow of the gas-phase refrigerant through the bypass pipe based on a comparison between the first refrigerant flow velocity and the second refrigerant flow velocity.