1. An image processing device, comprising: data receiving means for receiving a plurality of sets of digital input signals for a plurality of pixels that constitute a desired image; subject area setting means for setting a subject area that is formed from several pixels that include a subject pixel to be processed and that are arranged adjacent to one another, the subject pixel being located at a center of the subject area; pixel extracting means for extracting, from the subject area, a first predetermined number of pixels while remaining a second predetermined number of pixels not extracted in the subject area, each of the first predetermined number and the second predetermined number being greater than one, the total number of the first and second predetermined numbers being equal to the total number of the several pixels within the subject area, brightness of the extracted pixels being smaller than or equal to that of the non-extracted pixels; connected-state judging means for judging whether or not the extracted pixels are connected; and fine-line part determining means for determining that the subject pixel is in a fine-line part when the extracted several pixels are judged to be in a connected state.