1. An image forming system for forming a subject image, which has been formed on a recording medium, and which is observed by superposing an optical member which has a one-dimensional periodic structure, comprising: image acquisition means for acquiring an image sequence of the subject image; image synthesizing means for synthesizing a stripe image in such a manner that pixels having identical coordinates in each image of the image sequence will be arrayed as adjacent pixels; error diffusion quantization means for converting the stripe image to a multivalued image by reducing a number of tones through the error diffusion method; binarization means for converting the multivalued image to a binary dot pattern through a density pattern method; and image forming means for forming the binary dot pattern on a recording medium, wherein said binarization means switches among the plurality of dot patterns, which are the same level as each other, sequentially in accordance with a pixel position, in the multivalued image, in a direction perpendicular to a direction of repetition of the periodic structure of the optical member, so as to, when the optical member is superimposed, reduce occurrence of fringes by shortening a period of a binary dot pattern in the direction of repetition of the periodic structure from a period of a binary dot pattern in the direction perpendicular to the direction of repetition of the periodic structure, thereby converting the multivalued image to a binary dot pattern.