A scanning transmission electron microscope that scans a specimen with an electron probe to acquire an image. The scanning transmission electron microscope includes: an optical system which includes a condenser lens and an objective lens; an imaging device which is arranged on a back focal plane or a plane conjugate to the back focal plane of the objective lens and which is capable of photographing a Ronchigram; and a control unit which performs adjustment of the optical system. The control unit is configured or programed to: acquire an image of a change in a Ronchigram that is attributable to a change in a relative positional relationship between the specimen and the electron probe; and determine a center of the Ronchigram based on the image of the change in the Ronchigram.