1. An image compression device comprising: block division portion that divides an entire image into blocks of n×m pixels (where n and m are real numbers equal to or greater than 4); first inclined plane calculation portion that finds parameters that specify a first inclined plane that approximates said block, from said block pixel values; error amount calculation portion that calculates n×m pieces of error amounts by calculating the difference between said first inclined plane and each pixel value in said block; second inclined plane calculation portion that in the case where a pixel having said error amount above a predetermined threshold value is present, finds parameters that specify a second inclined plane only from the pixel values having the error amount above the predetermined threshold value; and encoding portion that performs reversible encoding for the parameters that specify said first and second inclined planes