A turbine blade designing method is for designing a turbine blade formed using a metal material in which creep including diffusion creep and dislocation creep occurs by heating. The turbine blade designing method includes: acquiring temperature distribution data relating to temperature distribution in the turbine blade to be heated; acquiring creep strength distribution data relating to distribution of the creep strength required for the turbine blade to be heated; from the correlation data, based on the temperature distribution data and the creep strength distribution data, setting the crystal grain size of a high-temperature portion that is the diffusion creep temperature range of the turbine blade to a size coarser than the reference crystal grain size, and setting the crystal grain size of a low-temperature portion that is the dislocation creep temperature range of the turbine blade to a size finer than the reference crystal grain size.
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