A pattern image for registration correction is formed in a reference color and an arbitrary color to be corrected. The pattern image for registration correction is formed a number or times n1 or a number of times n2. If the total number of image forming sections is S; the length of sheets is L; the sheet-to-sheet distance is d; and the belt conveying distance corresponding to one revolution of a drive roller is A, n1 satisfies ((L+d).times.(S-1)/A).times.n1=N1 (where n1 is an integer and N1 is an approximate integer), and n2 satisfies ((L+d)/A).times.n2=N2 (where n2 is an integer, and N2 is an approximate integer).