A sample milling apparatus includes an ion source, a swinging mechanism for swinging a sample, a positioning camera for bringing a target milling position on the sample into coincidence with the impact point of an ion beam, and a display section for displaying an image captured by the positioning camera. The adjustment method starts with observing the trace of the impinging ion beam left on the sample with the positioning camera while the position of the positioning camera is held relative to the swing axis of the swinging mechanism and capturing an observation image. Then, a display image to be displayed on the display section is extracted from the observation image based on the position of the trace, thus bringing the beam impact point and the position of the field of view of the display image into coincidence.