Costs are reduced, by decreasing the number of encoders used to compress images when compressing two or more images at different compression rates. A region of interest is detected within a processing target image, and a region of interest image is generated. A reduced image is generated by reducing the size of the processing target image. The reduced image and the region of interest image are multiplexed in an image space to generate a multiplex image. The multiplex image is compressed to generate compressed image data.