To provide a condensate flow rate control device and a control method for a power plant which improve responsiveness to frequency fluctuations or requested load changes and can suppress frequency fluctuations with precision or improve precision with which output power conforms to requested load instructions. A power plant to which a condensate flow rate control device is adapted includes a deaerator to which condensate generated by a condenser is supplied via a deaerator water level adjustment valve and into which bleed steam from a steam turbine is introduced. The condensate flow rate control device has a water level adjustment unit for executing condensate flow rate control, wherein the water level adjustment unit adjusts pressure in a condensate flow path extending from the deaerator water level adjustment valve to the deaerator so that inputted frequency fluctuations are suppressed or an output value of a generator conforms to inputted requested load changes.