A focusing device including: an optical system that includes a focusing lens; an image sensor that captures a subject image formed via the optical system and outputs an image signal containing a plurality of color components; an edge detection unit that detects an edge in correspondence to each color component among the plurality of color components contained in the image signal; a differential value calculation unit that calculates a differential value of an edge strength of the edge detected by the edge detection unit, in correspondence to the each color component; and a decision-making unit that determines a moving direction along which the focusing lens is to move for purposes of focus adjustment of the optical system based upon the differential value calculated by the differential value calculation unit.