This study proposes a numerical and analytical coupling method for simulating unsteady steam flow through three-stage stator and rotor blade rows of an intermediate-pressure steam turbine operating in a power plant by considering both the secular-changed blades and the leakage flows in labyrinth seals. The flow simulation was based on the in-house CFD code developed by Tohoku University, whereas the leakage flows in labyrinth seals were predicted by an analytical method. The effects of secular-changed blades and leakage flows on unsteady steam flows are discussed on the basis of time-dependent solutions. Next, the total performances obtained under conditions using manufactured and secular-changed blades both with and without leakage flows were compared with the actual data measured in operation. The obtained results indicate that the increase in static temperature at the outlet of actual intermediate-pressure steam turbines during a long-time operation is obviously caused by the secular change of blades. This paper presents the first results based on a numerical approach to clarify the temperature increase in a working intermediate-pressure steam turbine.