An image-forming device includes: a data conversion unit; an isolated-dot size determining unit; and an image-forming unit. The data conversion unit converts, for each of a plurality of pixels in an image to be formed, original image data indicative of a multilevel density into dot data indicative of whether or not a dot should be formed. The isolated-dot size determining unit determines the sizeof an isolated dot according to a condition. The image-forming unit forms an image on a recording medium by selectively forming a dot for each pixel based on the dot data, while forming the isolated dot with the determined size.