Since environmental problems are global issues, high thermal efficiency combined-cycle power plants are being expected to contribute to energy resource saving. Mitsubishi Heavy Industries, Ltd. Installed a large-capacity, high-efficiency combined-cycle power plant equipped with the world's first 1 450°C class turbine inlet temperature gas turbines for Unit No.4 -1 at the Higashi Niigata Thermal Power Station of Tohoku Electric Co., Inc. On the basis of higher heating value, it provides 50.6% thermal efficiency at the generator terminal, and began commercial operation in July 1999. Its successful operation and high reliability were confirmed in its first inspection after 10000 hours of operation.