PURPOSE: To compress the image data with high picture quality and at the fixed compression rate by selecting the reversible compression processing or the non-reversible compression processing based on the relationship between the quantity of compressed data undergone the reversible compression of a block and a prescribed quantity of data.

CONSTITUTION: An image processor which processes the image data for each block is provided with a reversible compressing means 3 which reversibly compress the blocks and an ADCT compressing means 4 which non-reversibly compress the image data. Then the block undergoes the reversible compressing processing with D≤L and the non-reversible compression processing with D>L respectively (D: the compressed data quantity obtained with the reversible compression of the block; L: the prescribed data quantity). Then the result obtained by expanding the compressed data as the reversible compressed data or the result obtained by expanding the compressed data as the non-reversible compressed data is selected baded on the input information. Thus, the original image data are restored. As the result, the images produced by a computer can be reversibly compressed and the picture quality of the natural images can be improved with the non-reversible compression processing.