A steam turbine casing position adjusting apparatus comprising: an inner casing and an outer casing; a rotor; the inner casing being disposed coaxially with the rotor with respect to a central axis while the outer casing is disposed so as to surround an outer side of the inner casing; a first actuator and a second actuator (14, 15) that move the inner casing in an axial direction with respect to the rotor, wherein each of the first actuator and the second actuator comprises a cylinder that extends in the axial direction, a piston that reciprocates in the axial direction and a rod that is fixed to one end surface of the piston and that advances and recedes in the axial direction; wherein each of the first actuator and the second actuator (14, 15) is disposed radially outside an outer peripheral surface forming the outer casing, characterized in that the actuators (14, 15) are disposed on opposite sides of the central axis and not on an axial end surface of the inner casing, the apparatus further comprising: at least three displacement sensors for measuring relative axial distances (11, 12, 13, 73, 74, 75, 76, 77, 78) that are fixed to the inner casing or a ground on which the outer casing is installed; a calculator that calculates a thermal elongation difference of the rotor in the axial direction with respect to the inner casing and an angle of inclination of the rotor with respect to the inner casing, based on data sent from the sensors; a controller that controls the rods of the first actuator and the second actuator (14, 15) to advance and recede such that the relative position relation between the inner casing and the rotor is not changed by cancelling the thermal elongation difference and the angle of inclination calculated by the calculator. an inner casing and an outer casing; a rotor; the inner casing being disposed coaxially with the rotor with respect to a central axis while the outer casing is disposed so as to surround an outer side of the inner casing; a first actuator and a second actuator (14, 15) that move the inner casing in an axial direction with respect to the rotor, wherein each of the first actuator and the second actuator comprises a cylinder that extends in the axial direction, a piston that reciprocates in the axial direction and a rod that is fixed to one end surface of the piston and that advances and recedes in the axial direction; wherein each of the first actuator and the second actuator (14, 15) is disposed radially outside an outer peripheral surface forming the outer casing, characterized in that the actuators (14, 15) are disposed on opposite sides of the central axis and not on an axial end surface of the inner casing, the apparatus further comprising: at least three displacement sensors for measuring relative axial distances (11, 12, 13, 73, 74, 75, 76, 77, 78) that are fixed to the inner casing or a ground on which the outer casing is installed; a calculator that calculates a thermal elongation difference of the rotor in the axial direction with respect to the inner casing and an angle of inclination of the rotor with respect to the inner casing, based on data sent from the sensors; a controller that controls the rods of the first actuator and the second actuator (14, 15) to advance and recede such that the relative position relation between the inner casing and the rotor is not changed by cancelling the thermal elongation difference and the angle of inclination calculated by the calculator. at least three displacement sensors for measuring relative axial distances (11, 12, 13, 73, 74, 75, 76, 77, 78) that are fixed to the inner casing or a ground on which the outer casing is installed; a calculator that calculates a thermal elongation difference of the rotor in the axial direction with respect to the inner casing and an angle of inclination of the rotor with respect to the inner casing, based on data sent from the sensors; a controller that controls the rods of the first actuator and the second actuator (14, 15) to advance and recede such that the relative position relation between the inner casing and the rotor is not changed by cancelling the thermal elongation difference and the angle of inclination calculated by the calculator.