1. A method of generating halftone dots in a plurality of one-dot areas in a halftone dot image for use in color printing in response to an image signal, said one-dot area being an area in which one halftone dot is to be formed, said method comprising the steps of: (a) providing threshold values to make isolated dots in the halftone dot image to be dispersed in an irregular manner when a level of said image signal is between a first signal level and a second signal level, said isolated dots being halftone dots each consisting of one pixel separated from other halftone dots, said first signal level being a level at which one isolated dot is formed in each of said plurality of one-dot areas, said second signal level being a level at which no isolated dots are formed in said plurality of one-dot areas; and (b) comparing said threshold values with said image signal, thereby generating a halftone dot signal representing said halftone dots, wherein each isolated dot is one of an isolated lit-up dot and an isolated non-lit-up dot, and each one-dot area includes a plurality of pixels.