There is an increasing need for high-precision gear machining to meet customer demand for low-noise and low-vibration planetary gear systems. Mitsubishi Heavy Industries, Ltd. (MHI) has been developing high-precision gear-cutting technologies for use before heat treatment in the production of ring (internal) gears, one of the components of planetary gear systems. However, high-precision high-efficiency gear grinding is also essential after heat treatment to eliminate distortion and improve gear accuracy. In this paper, we report on the development of the Mitsubishi ZI20A grinding machine, the world's first internal gear grinder for use in volume production.