A reflected light image and a fluorescent light image of an observed region are acquired with an imaging system. A gradation level of the reflected light image is set on the basis of a feature value of the acquired reflected light image. The focus of the imaging system is controlled in accordance with the set gradation level. The fluorescent light image is corrected using a focus-controlled reflected light image of the observed region. Accordingly, the image of an object in the observed region from which the fluorescent light is generated is sharpened.