To perform the planned maintenance of a hot member considering the operation as the entire unit by simply grasping the service life of the hot member of a steam turbine unit.

Curves are prepared on the relationship between the elapsed year from the start of the operation of the steam turbine unit to the present time and the operating time, and the relationship between the year from the present time of the turbine unit determined from the future operation plan and the operating time. The range of the service life is obtained for each of a plurality of hot members constituting the steam turbine unit in which the probability to the creep crack generating service life reaches a predetermined value. By plotting the range of the service life of each hot member on the curves, the temporal distribution of the range of the service life of each hot member is tabulated, and the maintenance is controlled on each hot member to the macroscopic service life as the entire unit.