A turbine design method that can easily construct a turbine stage structure in which an unsteady force acting on a rotor blade can be reduced and degradation in performance and increase in rotor shaft length can be prevented is provided. For example, unsteady forces and exciting forces by the potential field interaction acting on the rotor blade are respectively obtained by the viscous flow solution (21) and the inviscid flow solution (22) with respect to a plurality of distances between stator and rotor blades, exciting forces by the wake interaction acting on the rotor blade are obtained from a difference between a viscous flow solution result and an inviscid flow solution result, the exciting forces by the potential field interaction and the wake interaction are mathematically expressed (23) as functions of the distance between stator and rotor blades, the unsteady force acting on the rotor blade when the distance between stator and rotor blades is an arbitrary value is calculated (24) based on the mathematically-expressed exciting forces by the potential field interaction and the wake interaction, and the distance between stator and rotor blades is determined (30) based on a calculation result.
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