High thermal efficiency power plants are being installed world-wide to meet requirements for energy resource saving and global environmental conservation. Advanced combined-cycle power plants are attracting attention due to their high thermal efficiency. Mitsubishi Heavy Industries, Ltd., installed a large-capacity, high-efficiency 1 090 MW combined-cycle power plant as Unit No.3 in the Higashi Niigata Thermal Power Station of Tohoku Electric Co., Inc. in 1984. We also installed an advanced combined-cycle power plant using M701G gas turbines as Unit No.4 at the same site. In July 1999, 805 MW Unit No.4-1 began commercial operation, combining excellent performance with high reliability.