A system calculates axial deformation of a turbine rotor. The system includes: an input part that sets measurement points of a rotor disk that constitutes the turbine rotor; a deflection amount detection part that derives radial displacement amounts of the turbine rotor for at least four or more measurement points along an outer surface in a circumferential direction of the turbine rotor based on measured values measured by a displacement gage; a storage part that stores the radial displacement amounts derived by the deflection amount detection part and measured angles at the measurement points; and a calculation part that calculates misalignment data of a most probable circle of the turbine rotor based on data stored in the storage part. At least four or more measurement points are set in the circumferential direction of the turbine rotor. The calculated circle is calculated from the displacement amount and the measured angle measured at each measurement point. The most probable circle is determined from the calculated circle, and the misalignment data of the most probable circle is calculated from the most probable circle, thereby allowing axial deformation to be calculated by a simpler method than conventional methods.