Gear transmission is used in a wide field of general industries. In among them, reduction gear unit for a gas turbine - generator is a large capacity and high- speed rotation. With the increase in the size of the gas turbine, the reduction gear also required a large capacity. Hitachi Nico Transmission Co., Ltd. (herein, Hitachi Nico) is providing a reduction gear unit to the domestic gas turbine companies. The uses of the gas turbine are for power of power generator, for general industrial and oil & gas field etc. Therefore, the reduction gear unit has been required high reliability and high efficiency. In this paper, large capacity gear unit for gas turbine is described.