To provide an image processor with a configuration including a plurality of companding units each having an exclusive image memory that properly selects the companding unit and the image memory for generating thumbnail pictures to thereby prevent a thumbnail picture display processing from hindering the progress of a job with higher priority.

When compressed image data being a source of thumbnail pictures exist in a second image memory 31, the second companding unit 33 expands the image data to generate the thumbnail pictures. When the compressed image data being the source of the thumbnail pictures exist, on the other hand, in the other place (a first image memory 21 or a hard disk unit 47) than the second image memory 31, a copy of the image data is generated in the second image memory 31, the second companding unit 33 expands the image data of the copy destination to generate the thumbnail pictures. That is, even when the original image data exist anywhere, the thumbnail pictures are generated by using the second companding unit 33 and the second image memory 31.