An outline definition apparatus includes an edge detection unit detecting pixels at edge positions, each of the pixels at the edge positions having a data value between a value representing a first level and a value representing a second level, a pixel extraction unit extracting the pixels at the edge positions and extracting pixels in the vicinity thereof, a boundary-line generation unit generating a boundary line indicating a boundary between a region of the first level and a region of the second level in each of the pixels at the edge positions, and a link-processing unit obtaining an outline indicating a boundary between a region of the first level and a region of the second level in the still image by linking the boundary lines which are generated in the boundary-line generation unit and which are generated for the pixels which are located at the edge positions.