Each of moire elements which are repeat elements of first-order moire appearing by overlaying halftone images of first and second color components is positioned at one of candidate positions arranged at a moire pitch Pm in first and second moire directions orthogonal to each other. In a matrix area of a third color component, dot centers are arranged in a first tilt direction tilted relatively to the first moire direction by an angle of arctan((Pm)/(Pm)) and a second tilt direction orthogonal to the first tilt direction at a pitch of the square root of ((Pm2)+(Pm2)), and a threshold matrix of the color component is generated so that a halftone dot grows from each dot center in accordance with increase of gray level of the color component of the original image. Thus, it is possible to suppress second-order moire appearing in overlaying halftone images of the first to third color components.