1. An image decoder which applies a decoding process to image data comprising: means for applying a variable length decoding process to a first image data; means for applying an inverse quantizing process to said first image data; means for determining a mode of discrete cosine transform of said first image data; inverse discrete cosine transform means for obtaining a second image; and wherein said inverse discrete cosine transform means transform said first image data into said second image having a lower degree than said first image data when said mode is a field mode which performs said discrete cosine transform by each field, and transform said first image data into said second image with keeping field information when said mode is a frame mode which performs said discrete cosine transform by each frame.