Abstract: An image signal encoding apparatus for encoding an image signal in which image of one picture plane consisting of a plurality of pixel data are divided into a plurality of blocks each consisting of a predetermined number of pixel data. For each of the blocks, a pair of reference value data regarding the maximum and minimum values of the levels of the pixel data constructing the block are formed. On the basis of the formed reference value data, encoded date is formed by encoding each of the pixel data of the block. Decoded data is formed by decoding the encoded data formed on the basis of the reference value data. By comparing each of the pixel data of the block with the decoded data, errors are detected. The reference value data are corrected in accordance with the result of the detection. Thus, the encoding errors are reduced and it is possible to perform the encoding with less deterioration in picture quality of the image signal.