1. An image capturing apparatus that carries out autofocus control by driving a focus lens, comprising: a detection unit which detects a region of a particular object from a captured image; a setting unit which sets a focus detection area based on detection results obtained by the detection unit; a generating unit which generates an AF evaluation value based on a component of a predetermined frequency band contained in the focus detection area of a captured image; a determination unit which determines an in-focus position search direction based on a change in a size of the region of the particular object detected by the detection unit; and a control unit which determines an in-focus position based on a plurality of AF evaluation values obtained at different positions of the focus lens by driving the focus lens to the different positions based on the in-focus position search direction determined by the determination unit if it is determined that the change in the size of the region of the particular object is equal to or larger than a predetermined threshold.