A polygon machining method whereby first polygon machining using a polygon cutter is carried out on a workpiece, followed by machining using tools other than the polygon cutter, then second polygon machining using the polygon cutter again. The polygon machining method comprises: a first polygon machining step in which a main axis and a tool main axis are synchronously rotated such that the rotation speed of the main axis and the tool main axis are at a ratio required for the first polygon machining, and polygon machining is carried out; a machining step in which the ratio is changed to a second synchronization ratio such that the main axis rotates at a rotation speed required for machining after the first polygon machining, the main axis and the tool main axis are synchronously rotated, and machining is carried out on the workpiece that has received the first polygon machining; and a second polygon machining step in which the main axis and the tool main axis are synchronously rotated such that the rotation speed of the main axis and the tool main axis are at a ratio required for the second polygon machining, and polygon machining is carried out.