PURPOSE: To perform a color dot process by providing a means which decides the screen angle of a pattern matrix constituting a dot picture and the density of the dots by turning on the bit of the pattern matrix.

CONSTITUTION: The screen angle θ of a pattern matrix is set by arranging the same numerals as numbers of dots constituting the pattern matrix on two sides of a right-angled triangle on both sides of the right angle so that they can maintain a fixed relation and, at the same time, the pattern matrix is set to the density corresponding to the number of dots by turning on several kinds of the same number, for example, 1 of dots. Then a block copy for a plate is produced based on the pattern matrix and dots of an arbitrary screen angle and density are printed by using the plate produced from the block copy. Therefore, dots of an arbitrary screen angle and density can be set easily and the color dot process can be processed with a computer.