To provide a rotor material preferable for a steam turbine of which main steam temperature is 675.degree. C. or more, particularly exceeding 700.degree. C., and a steam turbine plant having a rotor formed by the material, the invention provides a steam turbine plant including a very-high-pressure turbine of which steam inlet temperature is 675 to 725.degree. C. and steam outlet temperature is 650.degree. C. or less, a high-pressure turbine, and a medium-low-pressure turbine, wherein a rotor of the very-high-pressure turbine is formed from a forged material of NiFe-base alloy containing: 14 to 18 weight % Cr, 15 to 45 weight % Fe, 1.0 to 2.0 weight % Al, 1.0 to 1.8 weight % Ti, C and N of which the sum is 0.05 or less weight %, and Nb in the range specified by the formula: 3.5-(Fe weight %)/20<(Nb weight %)<4.5-(Fe weight %)/20.

 
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