An auto focus apparatus configured to be installed in a microscope including an objective lens, the auto focus apparatus comprising: a light source which irradiates an auxiliary light for focal point detection onto an object; an illumination optical system which forms an optical image of the auxiliary light from the light source on the object via the objective lens; an imaging optical system which forms an image of the reflected auxiliary light reflected from the object via the objective lens; a photoelectric converter which is disposed in an imaging position of the reflected image by the auto focus imaging optical system, and detects the reflected image of the auxiliary light; and irradiation optical path change means (135, 261), disposed on a common optical path of the auto focus illumination optical system and the auto focus imaging optical system, for shifting the irradiation optical path of the auxiliary light directed to the objective lens and changing the imaging position of the auxiliary light on the object; characterized in that the auto focus apparatus further comprises an offset optical system which is configured to adjust an offset of a focal position of an observation optical system of the microscope, and the imaging position of the auto focus illumination optical system, and which is disposed on the optical path of the auto focus illumination optical system, and in that the irradiation optical path change means (135, 261) is configured to be disposed between the objective lens and the offset optical system and move an irradiation position of the auxiliary light on the object to an area within an observation field of the objective lens and outside an observation field of a camera of the microscope. a light source which irradiates an auxiliary light for focal point detection onto an object; an illumination optical system which forms an optical image of the auxiliary light from the light source on the object via the objective lens; an imaging optical system which forms an image of the reflected auxiliary light reflected from the object via the objective lens; a photoelectric converter which is disposed in an imaging position of the reflected image by the auto focus imaging optical system, and detects the reflected image of the auxiliary light; and irradiation optical path change means (135, 261), disposed on a common optical path of the auto focus illumination optical system and the auto focus imaging optical system, for shifting the irradiation optical path of the auxiliary light directed to the objective lens and changing the imaging position of the auxiliary light on the object; characterized in that the auto focus apparatus further comprises an offset optical system which is configured to adjust an offset of a focal position of an observation optical system of the microscope, and the imaging position of the auto focus illumination optical system, and which is disposed on the optical path of the auto focus illumination optical system, and in that the irradiation optical path change means (135, 261) is configured to be disposed between the objective lens and the offset optical system and move an irradiation position of the auxiliary light on the object to an area within an observation field of the objective lens and outside an observation field of a camera of the microscope.