A steam turbine system comprising: a steam turbine having a casing into which steam is fed from an outside of the casing, and a rotating shaft that is provided within the casing so as to be rotatable around a central axis (Ac); a deaerator that is connected to the steam turbine and configured to deaerate leaked steam that has leaked from a gap between the casing and the rotating shaft to the outside of the casing; a vacuum pump that is connected to the deaerator and configured to lower a pressure within the deaerator; and a shaft sealing device that is connected to the deaerator and is configured to seal the gap between the casing and the rotating shaft, wherein the shaft sealing device has a seal member having a seal body that is configured to seal a portion between the seal body and an outer peripheral surface of the rotating shaft, and a pressure-receiving part that is connected to the seal body and is configured to move the seal body in a radial direction (Dr) of the rotating shaft, a housing that holds the pressure-receiving part in an accommodating recess formed therein so as to be movable in the radial direction (Dr), a biasing member that is configured to bias the pressure-receiving part toward an inside of the radial direction (Dr), and a negative-pressure introduction part that is configured to make a pressure inside the accommodating recess lower than a pressure outside the housing, characterized by further comprising: a control device that is configured to control the negative-pressure introduction part so as to lower a pressure within the accommodating recess depending on an operation of the steam turbine, wherein the control device is configured to cause the negative-pressure introduction part to lower the pressure within the accommodating recess in a case where a pressure of the steam within the casing is equal to or higher than a predetermined pressure. a steam turbine having a casing into which steam is fed from an outside of the casing, and a rotating shaft that is provided within the casing so as to be rotatable around a central axis (Ac); a deaerator that is connected to the steam turbine and configured to deaerate leaked steam that has leaked from a gap between the casing and the rotating shaft to the outside of the casing; a vacuum pump that is connected to the deaerator and configured to lower a pressure within the deaerator; and a shaft sealing device that is connected to the deaerator and is configured to seal the gap between the casing and the rotating shaft, wherein the shaft sealing device has a seal member having a seal body that is configured to seal a portion between the seal body and an outer peripheral surface of the rotating shaft, and a pressure-receiving part that is connected to the seal body and is configured to move the seal body in a radial direction (Dr) of the rotating shaft, a housing that holds the pressure-receiving part in an accommodating recess formed therein so as to be movable in the radial direction (Dr), a biasing member that is configured to bias the pressure-receiving part toward an inside of the radial direction (Dr), and a negative-pressure introduction part that is configured to make a pressure inside the accommodating recess lower than a pressure outside the housing, a control device that is configured to control the negative-pressure introduction part so as to lower a pressure within the accommodating recess depending on an operation of the steam turbine, wherein the control device is configured to cause the negative-pressure introduction part to lower the pressure within the accommodating recess in a case where a pressure of the steam within the casing is equal to or higher than a predetermined pressure.