A scanning transmission electron microscope, comprising: an electron optical system that includes an aberration corrector; and a control unit that controls the aberration corrector, the control unit performing: processing of acquiring a first scanning transmission electron microscope image and a second scanning transmission electron microscope image photographed at mutually different defocus amounts; processing of calculating a first auto-correlation function that is an auto-correlation function of the 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 calculating a second auto-correlation function that is an auto-correlation function of the second scanning transmission electron microscope image; processing of acquiring a second intensity profile along a straight line that passes through a center of the second auto-correlation function; processing of obtaining a position of an inflection point that is closest to the center of the second auto-correlation function in the second intensity profile and adopting an intensity at the position as a second 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 the second reference intensity in the second auto-correlation function; and processing of controlling the electron optical system based on the aberration coefficient. an electron optical system that includes an aberration corrector; and a control unit that controls the aberration corrector, the control unit performing: processing of acquiring a first scanning transmission electron microscope image and a second scanning transmission electron microscope image photographed at mutually different defocus amounts; processing of calculating a first auto-correlation function that is an auto-correlation function of the 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 calculating a second auto-correlation function that is an auto-correlation function of the second scanning transmission electron microscope image; processing of acquiring a second intensity profile along a straight line that passes through a center of the second auto-correlation function; processing of obtaining a position of an inflection point that is closest to the center of the second auto-correlation function in the second intensity profile and adopting an intensity at the position as a second 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 the second reference intensity in the second auto-correlation function; and processing of controlling the electron optical system based on the aberration coefficient. processing of acquiring a first scanning transmission electron microscope image and a second scanning transmission electron microscope image photographed at mutually different defocus amounts; processing of calculating a first auto-correlation function that is an auto-correlation function of the 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 calculating a second auto-correlation function that is an auto-correlation function of the second scanning transmission electron microscope image; processing of acquiring a second intensity profile along a straight line that passes through a center of the second auto-correlation function; processing of obtaining a position of an inflection point that is closest to the center of the second auto-correlation function in the second intensity profile and adopting an intensity at the position as a second 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 the second reference intensity in the second auto-correlation function; and processing of controlling the electron optical system based on the aberration coefficient.