An autofocus device comprises: a computation unit which receives an observation image from an observation unit including a focusing mechanism and performs a focus contrast computation on the observation image to acquire a focal position； a region setting unit which sets a portion of the observation region for the observation unit as a first focal region to be subjected to the focus computation, and changes the first focal region to set a second focal region when a focal position cannot be acquired from the focus computation on the first focal region； and a drive control unit which drives the focus mechanism on the basis of the focal position acquired by the computation unit.
