Provided is a processing method capable of accurately grasping a removal rate and coping with processing of an optical device or an optical device molding die, for which high shape accuracy is required. An ion beam as a tool is caused to scan a workpiece as a dummy work, and a dwell time of the tool at this time is changed based on a linear function with respect to a scanning position thereof and a removal rate with respect to the continuously changed dwell time is grasped based on an actually formed removal shape. The dwell time of the tool is changed based on the linear function with respect to the scanning position, and hence a removal rate based on the actual removal shape can be acquired continuously from a case where a scanning speed is fast to a case where the scanning speed is slow.