A pressure increasing system comprising: a plurality of stages of compressors configured to compress a gas to a target pressure higher than a critical pressure; intermediate coolers that are provided between the plurality of stages of compressors and cool the gas discharged from a compressor in a preceding stage; a subcooler that cools the gas; a bypass line which is connected to an inlet of the final stage compressor and through which the gas in the inlet of the final stage compressor is extracted and supplied to the subcooler as a refrigerant; a flow rate adjusting valve which is provided on the bypass line and by which the extracted gas is depressurized upstream from the subcooler ; a sensor configured to measure at least one of a temperature and a pressure of the gas in the inlet of the final stage compressor ; and a control unit configured to regulate a degree of opening of the flow rate adjusting valve based on a value measured by the sensor so that at least one of a temperature and a pressure of the gas in the inlet of the final stage compressor remains constant; wherein: the subcooler is provided between the final stage compressor and a compressor in a stage before the final stage; a pressure of the gas in the inlet of the final stage compressor is the critical pressure or higher than the critical pressure; the final stage compressor is configured to compress the gas such that a temperature of the gas at an outlet of the final stage compressor is higher than the critical temperature. a plurality of stages of compressors configured to compress a gas to a target pressure higher than a critical pressure; intermediate coolers that are provided between the plurality of stages of compressors and cool the gas discharged from a compressor in a preceding stage; a subcooler that cools the gas; a bypass line which is connected to an inlet of the final stage compressor and through which the gas in the inlet of the final stage compressor is extracted and supplied to the subcooler as a refrigerant; a flow rate adjusting valve which is provided on the bypass line and by which the extracted gas is depressurized upstream from the subcooler ; a sensor configured to measure at least one of a temperature and a pressure of the gas in the inlet of the final stage compressor ; and a control unit configured to regulate a degree of opening of the flow rate adjusting valve based on a value measured by the sensor so that at least one of a temperature and a pressure of the gas in the inlet of the final stage compressor remains constant; wherein: the subcooler is provided between the final stage compressor and a compressor in a stage before the final stage; a pressure of the gas in the inlet of the final stage compressor is the critical pressure or higher than the critical pressure; the final stage compressor is configured to compress the gas such that a temperature of the gas at an outlet of the final stage compressor is higher than the critical temperature. the subcooler is provided between the final stage compressor and a compressor in a stage before the final stage; a pressure of the gas in the inlet of the final stage compressor is the critical pressure or higher than the critical pressure; the final stage compressor is configured to compress the gas such that a temperature of the gas at an outlet of the final stage compressor is higher than the critical temperature. <EOS>