Provided is an internal gear machining method capable of achieving improvement of machining accuracy and extension of the tool life by increasing a slip velocity. For this purpose, internal gear machining is performed on a workpiece (W) by synchronously rotating the workpiece (W) and a barrel-shaped threaded grinding wheel (11) in mesh with each other. The workpiece (W) is rotatable about a workpiece rotation axis (C1), and the barrel-shaped threaded grinding wheel (11) is rotatable about a grinding wheel rotation axis (B1) having a predetermined shaft angle (Σ) to the workpiece rotation axis (C1). In the internal gear machining method, the shaft angle (Σ) is set large based on the amount of change in a grinding pitch wheel pitch radius of the threaded grinding wheel (11).