A carbon dioxide recovery system, comprising: a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine; a boiler for producing steam to drive the turbines; a carbon dioxide recovery unit including a carbon dioxide absorber for allowing a carbon dioxide absorbent to absorb and reduce carbon dioxide in a combustion flue gas discharged from the boiler and a carbon dioxide regenerator for regenerating the carbon dioxide absorbent having absorbed carbon dioxide as a regenerated carbon dioxide absorbent; a first steam line through which low-pressure steam is fed from the intermediate-pressure turbine to the low-pressure turbine; a second steam line into which the low-pressure steam is branched from the first steam line; a first regulation valve disposed on the first steam line and being configured to be able to regulate an amount of steam of the low-pressure steam by an opening from 100% to 0%; a second regulation valve disposed on the second steam line and being configured to be able to regulate an amount of steam of the low-pressure steam by an opening from 0% to 100% depending on the amount of control provided to the first regulation valve; an auxiliary turbine coupled to the second steam line to recover power using the low-pressure steam being fed; a steam feed line through which exhaust steam discharged from the auxiliary turbine is employed to be fed as a source of heat to a reboiler which is used to regenerate the carbon dioxide absorbent having absorbed carbon dioxide in the carbon dioxide regenerator; a bleed line through which the low-pressure steam having been bled from the auxiliary turbine is fed to the low-pressure turbine; and a control device configured to provide control to drive the auxiliary turbine by controlling an opening of the first regulation valve from 100% to 0% and of the second regulation valve from 0% to 100% in carbon dioxide recovery and non-recovery modes in response to a variation in operation load of the boiler or the turbines while a pressure of the exhaust steam supplied to the reboiler is maintained to take on a permissible value for a reboiler optimum pressure (0.33 MPa ± 0.05 MPa) with the first regulation valve and the second regulation valve being operated in response to each other, when carbon dioxide is recovered. a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine; a boiler for producing steam to drive the turbines; a carbon dioxide recovery unit including a carbon dioxide absorber for allowing a carbon dioxide absorbent to absorb and reduce carbon dioxide in a combustion flue gas discharged from the boiler and a carbon dioxide regenerator for regenerating the carbon dioxide absorbent having absorbed carbon dioxide as a regenerated carbon dioxide absorbent; a first steam line through which low-pressure steam is fed from the intermediate-pressure turbine to the low-pressure turbine; a second steam line into which the low-pressure steam is branched from the first steam line; a first regulation valve disposed on the first steam line and being configured to be able to regulate an amount of steam of the low-pressure steam by an opening from 100% to 0%; a second regulation valve disposed on the second steam line and being configured to be able to regulate an amount of steam of the low-pressure steam by an opening from 0% to 100% depending on the amount of control provided to the first regulation valve; an auxiliary turbine coupled to the second steam line to recover power using the low-pressure steam being fed; a steam feed line through which exhaust steam discharged from the auxiliary turbine is employed to be fed as a source of heat to a reboiler which is used to regenerate the carbon dioxide absorbent having absorbed carbon dioxide in the carbon dioxide regenerator; a bleed line through which the low-pressure steam having been bled from the auxiliary turbine is fed to the low-pressure turbine; and a control device configured to provide control to drive the auxiliary turbine by controlling an opening of the first regulation valve from 100% to 0% and of the second regulation valve from 0% to 100% in carbon dioxide recovery and non-recovery modes in response to a variation in operation load of the boiler or the turbines while a pressure of the exhaust steam supplied to the reboiler is maintained to take on a permissible value for a reboiler optimum pressure (0.33 MPa ± 0.05 MPa) with the first regulation valve and the second regulation valve being operated in response to each other, when carbon dioxide is recovered.