In a scanning transmission electron microscope, a control unit performs: processing of calculating a first auto-correlation function that is an auto-correlation function of a first scanning transmission electron microscope image; processing of acquiring a first intensity profile along a straight line that passes through a center of the first auto-correlation function; processing of obtaining a position of an inflection point that is closest to the center of the first auto-correlation function in the first intensity profile and adopting an intensity at the position as a first reference intensity; processing of obtaining an aberration coefficient by fitting a first aberration function to an isointensity line that connects positions where intensity is equal to the first reference intensity in the first auto-correlation function and by fitting a second aberration function to an isointensity line that connects positions where intensity is equal to a second reference intensity in a second auto-correlation function; and processing of controlling an electron optical system based on the aberration coefficient.