First, an ion beam is applied to a workpiece to form a tapered hole the side wall of which is inclined. Next, the application of the ion beam is stopped, and then a material gas is introduced from the gas source to the upper surface of the workpiece from an oblique direction to cause gas molecules to be adsorbed to the upper surface of the workpiece and to the upper portion of the side wall of the hole. Next, introduction of the material gas is stopped, and then the ion beam is applied again to the region of the workpiece where the hole is formed. As a result, at the upper portion of the side wall of the hole, film formation occurs using the gas molecules as the material adsorbed to the side wall of the hole, and, at the bottom portion of the hole, etching of the workpiece occurs.