A steam turbine comprising: a rotor which has a rotor body extending along an axis, a plurality of stages of rotor blade rows, and a balance piston provided on one axial side of the plurality of stages of rotor blade rows; a casing which covers the rotor from an outside in a radial direction relative to the axis and forms, between the casing and the rotor, a plurality of blade chambers corresponding to the rotor blade rows, a first chamber on another axial side of the balance piston, and a second chamber on the one axial side of the balance piston; a thrust bearing which receives a thrust force applied to the rotor; a steam inlet through which steam is introduced into the first chamber, characterised by: a first pipe which connects the second chamber of the balance piston and one blade chamber of the plurality of blade chambers to each other; a first regulation valve which is provided in the first pipe; a second pipe which connects the second chamber of the balance piston and another blade chamber of the plurality of blade chambers to each other, another blade chamber having an internal pressure different from that of the one blade chamber; a second regulation valve which is provided in the second pipe; and a control device which controls the first regulation valve such that an internal pressure of the second chamber is approximately the same as an internal pressure of the one blade chamber based on the thrust force applied to the thrust bearing and which controls the second regulation valve such that the internal pressure of the second chamber is approximately the same as an internal pressure of the another blade chamber based on the thrust force applied to the thrust bearing, and wherein the control device estimates an exhaust flow rate of the steam turbine based on an operating point map which derives the exhaust flow rate of the steam turbine from an operating point of the steam turbine, and estimates the thrust force applied to the thrust bearing, based on the exhaust flow rate. a rotor which has a rotor body extending along an axis, a plurality of stages of rotor blade rows, and a balance piston provided on one axial side of the plurality of stages of rotor blade rows; a casing which covers the rotor from an outside in a radial direction relative to the axis and forms, between the casing and the rotor, a plurality of blade chambers corresponding to the rotor blade rows, a first chamber on another axial side of the balance piston, and a second chamber on the one axial side of the balance piston; a thrust bearing which receives a thrust force applied to the rotor; a steam inlet through which steam is introduced into the first chamber, characterised by: a first pipe which connects the second chamber of the balance piston and one blade chamber of the plurality of blade chambers to each other; a first regulation valve which is provided in the first pipe; a second pipe which connects the second chamber of the balance piston and another blade chamber of the plurality of blade chambers to each other, another blade chamber having an internal pressure different from that of the one blade chamber; a second regulation valve which is provided in the second pipe; and a control device which controls the first regulation valve such that an internal pressure of the second chamber is approximately the same as an internal pressure of the one blade chamber based on the thrust force applied to the thrust bearing and which controls the second regulation valve such that the internal pressure of the second chamber is approximately the same as an internal pressure of the another blade chamber based on the thrust force applied to the thrust bearing, and a first pipe which connects the second chamber of the balance piston and one blade chamber of the plurality of blade chambers to each other; a first regulation valve which is provided in the first pipe; a second pipe which connects the second chamber of the balance piston and another blade chamber of the plurality of blade chambers to each other, another blade chamber having an internal pressure different from that of the one blade chamber; a second regulation valve which is provided in the second pipe; and a control device which controls the first regulation valve such that an internal pressure of the second chamber is approximately the same as an internal pressure of the one blade chamber based on the thrust force applied to the thrust bearing and which controls the second regulation valve such that the internal pressure of the second chamber is approximately the same as an internal pressure of the another blade chamber based on the thrust force applied to the thrust bearing, and wherein the control device estimates an exhaust flow rate of the steam turbine based on an operating point map which derives the exhaust flow rate of the steam turbine from an operating point of the steam turbine, and estimates the thrust force applied to the thrust bearing, based on the exhaust flow rate.