Ion beam etching method and ion beam etching apparatus

To provide an ion beam etching method which enables a highly uniform IBE process even under a low-angle-incident static condition, without increase in the size of an apparatus. The ion beam etching method includes: changing a position of an opening portion with respect to a substrate; etching the substrate with an ion beam passing through the opening portion; and reducing a tilt angle as a center of a site where the ion beam is incident on the substrate moves away from the ion source.