1. An image processing system comprising a first image processing apparatus which records, on a recording medium, composite image information created by embedding invisible sub-information in visible main image information, and a second image processing apparatus which restores the sub-information from the composite image information recorded on the recording medium by the first image processing apparatus, the first image processing apparatus including a pre-processing unit which performs, for main image information, pre-processing corresponding to pixel formation processing for image recording in the first image processing apparatus, an embedding processing unit which creates composite image information by embedding sub-information in main image information in an invisible state using the main image information, the sub-information, and key information used to restore the sub-information, and a recording unit which records the composite image information created by the embedding processing unit on a recording medium, and the second image processing apparatus including an image input unit which inputs the composite image information from the recording medium on which the composite image information is recorded by the recording unit of the first image processing apparatus, a frequency component extracting unit which extracts a spatial frequency component unique to the key information from the composite image information input by the image input unit, and a reconstructing unit which reconstructs the sub-information from the spatial frequency component extracted by the frequency component extracting unit.