Steam turbine plants face increasingly strict market demand for high-temperature operation, streamlining, and short end delivery times. It is thus imperative that a welded-rotor meeting these requirements be developed. The welded rotor operates at high temperature and enables streamlining by using the material for most appropriate high- to low-pressure parts. The use of smaller materials will also enable a shift to larger capacity and shorter delivery time. Mitsubishi Heavy Industries, Ltd., used welded rotors at its Takasago combinedcycle plant (steam turbine output: 105 MW, main steam temperature: 566o C). The firm has been executing proving operations for a common steel welded rotor since March 1997 and a dissimilar steel welded rotor since April 1999. We introduce verification of work execution, strength, thermal stability, and inspection for the welded rotor and actual machine operation under fine conditions.