To provide a method of machining a gear for precisely machining the internal diameter of the gear to be machined, and also to provide a machining device of the same.

The virtual center of the gear to be machined W is positioned at a specific position on X-Y planes which intersect. A master gear 3 is meshed with the gear to be machined W, with the gear to be machined W positioned at the virtual center by a positioning means 2. The master gear 3 is energized toward the gear to be machined W by an energizing means 4 under a meshed condition. The gear to be machined W and the master gear 3 are relatively rotated by a rotation driving means 5 under an energized and meshed condition. The displacement of the master gear to the X-Y direction is detected by a displacement detection means 25 during the rotation. A shift quantity in the X-Y direction is grasped by a shift grasping means 26, and the shift quantity is corrected by a machining control means 27 to machine the internal diameter.