1. An image-forming device comprising: a data conversion unit converting, 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; an isolated-dot size determining unit that determines the size of an isolated dot according to a condition; and an image-forming unit forming 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.