An autofocus device includes an objective lens, an observation optical system, a driving mechanism that displaces the objective lens in an optical axis direction thereof, an illumination-optical-system optical path used to illuminate a measurement surface of a measurement object through the objective lens with light, and a pattern-projection-optical-system optical path. An electronic control shutter, a pattern projection plate having a predetermined pattern formed thereon, and a projection lens are provided in the pattern-projection-optical-system optical path.