An autofocus system includes: an imaging unit which captures an image of an object formed by an optical system; an autofocus portion which performs focus adjustment of the optical system on an autofocus area set in the image captured by the imaging unit so that the object within the autofocus area comes into focus; an AF area tracking processing portion which performs tracking processing to set a predetermined object within the autofocus area as a target to be tracked, detect a presence area of the target to be tracked in an image newly captured by the imaging unit and determine the presence area as a new autofocus area; a depth-of-field computing portion which calculates a depth of field based on information acquired from the optical system; and a control portion as defined herein.