Provided are high-pressure, medium-pressure, and low-pressure turbines; a boiler to generate steam for driving the turbines; a carbon dioxide recovery unit including an absorber that reduces carbon dioxide in combustion flue gas from the boiler by means of a carbon dioxide absorbent and a regenerator that regenerates an absorbent; a first auxiliary turbine that extracts steam from an inlet of the low-pressure turbine and recovers power by means of the steam thus extracted; a first steam delivery line to supply discharged steam from the first auxiliary turbine to a reboiler of the regenerator as a heat source; and a controller that controls driving of the first auxiliary turbine while keeping pressure of the discharged steam to be supplied to the reboiler within a tolerance range for the reboiler's optimum pressure corresponding to a fluctuation in an operation load of the boiler.