1. An image printing system which determines dot forming condition when printing an image containing a plurality of pixels by using dots of a plurality of sizes, comprising a multi-core processor for operating: a fine drawing detecting unit which detects a dot color fine drawing pixel having a dot color used for printing the image and positioned on a fine drawing portion having a drawing width equal to or smaller than a predetermined value; an edge area detecting unit which detects a dot color edge area pixel having a dot color used for printing the image and positioned at a distance equal to or shorter than a predetermined value from an edge of the image; a first dot allocating unit which allocates dots such that a dot of predetermined size in the dots of the plural sizes can be formed for the dot color fine drawing pixel during printing the image; and a second dot allocating unit which allocates dots such that a dot of a size determined according to the distance from the edge of the image in the dots of the plural sizes can be formed for the dot color edge area pixel not corresponding to the dot color fine drawing pixel during printing the image.