1. A method of generating halftone threshold data for color images of C, M, Y, K to reproduce a color image, comprising the steps of: setting dot cells made up of the halftone threshold data under output conditions including an output resolution of 2000 dpi or less and an output resolution/halftone screen period ratio to 8 dpi/lpi or less; setting the dot cells of at least the three color images of C, M, K under the condition of a non-rational tangent; and setting halftone screen periods and screen angles of said three color images of C, M, K such that a period and an angle of a primary moiré pattern produced when two of the three color images of C, M, K are superposed are substantially equal to a halftone screen period and a screen angle of the remaining one of the three color images of C, M, K.