A steam turbine plant comprising: a boiler configured to heat water to change the water into steam; a first turbine configured to be driven by the steam from the boiler; a reheater configured to heat the steam exhausted from the first turbine; a second turbine configured to be driven by the steam from the reheater; a third turbine configured to be driven by the steam exhausted from the second turbine; a carbon dioxide capturing system including an absorbing tower configured to cause an absorbing liquid to absorb carbon dioxide, a regeneration tower configured to cause the absorbing liquid to release the carbon dioxide, and a reboiler configured to heat the absorbing liquid with heat of the steam exhausted or extracted from the second turbine; a steam pipe configured to feed the steam exhausted from the second turbine to the third turbine; a first steam extraction pipe configured to send the steam exhausted or extracted from the second turbine to the reboiler; and a second steam extraction pipe configured to send the steam exhausted or extracted from the first turbine to the reboiler, the plant further comprising: a pressure detector configured to detect a pressure of the steam flowing through the steam pipe or the first steam extraction pipe; a pressure adjustment valve provided on the steam pipe; a controller configured to control the pressure adjustment valve based on the pressure detected by the pressure detector; a first valve provided on the first steam extraction pipe and configured to be controlled by the controller; and a second valve provided on the second steam extraction pipe and configured to be controlled by the controller. a boiler configured to heat water to change the water into steam; a first turbine configured to be driven by the steam from the boiler; a reheater configured to heat the steam exhausted from the first turbine; a second turbine configured to be driven by the steam from the reheater; a third turbine configured to be driven by the steam exhausted from the second turbine; a carbon dioxide capturing system including an absorbing tower configured to cause an absorbing liquid to absorb carbon dioxide, a regeneration tower configured to cause the absorbing liquid to release the carbon dioxide, and a reboiler configured to heat the absorbing liquid with heat of the steam exhausted or extracted from the second turbine; a steam pipe configured to feed the steam exhausted from the second turbine to the third turbine; a first steam extraction pipe configured to send the steam exhausted or extracted from the second turbine to the reboiler; and a second steam extraction pipe configured to send the steam exhausted or extracted from the first turbine to the reboiler, the plant further comprising: a pressure detector configured to detect a pressure of the steam flowing through the steam pipe or the first steam extraction pipe; a pressure adjustment valve provided on the steam pipe; a controller configured to control the pressure adjustment valve based on the pressure detected by the pressure detector; a first valve provided on the first steam extraction pipe and configured to be controlled by the controller; and a second valve provided on the second steam extraction pipe and configured to be controlled by the controller. a pressure detector configured to detect a pressure of the steam flowing through the steam pipe or the first steam extraction pipe; a pressure adjustment valve provided on the steam pipe; a controller configured to control the pressure adjustment valve based on the pressure detected by the pressure detector; a first valve provided on the first steam extraction pipe and configured to be controlled by the controller; and a second valve provided on the second steam extraction pipe and configured to be controlled by the controller.