The autofocus device for a sample observation device provided with an objective is provided with a photodetector, an autofocus optical system that includes a tube lens, that guides light to one of two regions of a pupil plane of the objective that has been divided into two, and that guides light that has passed through the other region after being reflected by a sample to the photodetector, and a light shielding member which is arranged between the tube lens and a light receiving plane of the photodetector and on which an aperture has been formed. The autofocus device satisfies 2.1