A large-capacity multiplexing system assuring image quality and robustness is provided. An image-processing apparatus multiplexing additional information on image information includes a first multiplexing unit that is configured to perform pseudo-halftone processing so that the image information is changed into a first quantization level of multi-level gradation and that is configured to multiplex first information of the additional information at the pseudo-halftone-processing time, and a second multiplexing unit that is configured to perform the pseudo-halftone processing so that the first quantization level subjected to the pseudo-halftone processing by the first multiplexing unit is changed into a second quantization level of gradation lower than the multi-level gradation and that is configured to multiplex second information of the additional information at the pseudo-halftone-processing time.