An image decoding system for decoding a digital image comprising: means for decoding each of blocks into which an image is divided; intra-prediction means for predicting a pixel value of a target block to be decoded by using a pixel value of an adjacent block; and prediction mode estimating means for estimating a prediction mode for the target block; wherein the infra-prediction means includes a vertical prediction mode using a prediction value being a pixel value of a first block located adjacent to and above the target block; a horizontal prediction mode using a prediction value being a pixel value of a second block located adjacent to the left side of the target block; a DC prediction mode using a prediction value being an average of the pixel values of the first and second blocks; a diagonal down/left prediction mode using the specified direction being diagonally downward to the left at a 45 degree angle from the horizontal; a diagonal down/right prediction mode using the specified direction being diagonally downward to the right at a 45 degree angle from the horizontal; a vertical right prediction mode using the predicting direction being specified by one pixel rightward and two pixels downward; a horizontal down prediction mode using the predicting direction being specified by two pixels rightward and one pixel downward; a vertical left prediction mode using the predicting direction being specified by one pixel leftward and two pixels downward, a horizontal up prediction mode using the predicting direction being specified by two pixels rightward and one pixel upward; the prediction modes are numbered with increasingly larger numbers, in order of the vertical prediction mode, the horizontal prediction mode, the DC prediction mode, the diagonal down/left prediction mode, the diagonal down/right prediction mode, the vertical right prediction mode, the horizontal down prediction mode, the vertical left prediction mode and the horizontal up prediction mode, and the prediction mode estimating means determines a prediction mode to have the lower mode number among the prediction mode of the first block and the prediction mode of the second block as the estimated prediction mode for the target block means for decoding each of blocks into which an image is divided; intra-prediction means for predicting a pixel value of a target block to be decoded by using a pixel value of an adjacent block; and prediction mode estimating means for estimating a prediction mode for the target block; wherein the infra-prediction means includes a vertical prediction mode using a prediction value being a pixel value of a first block located adjacent to and above the target block; a horizontal prediction mode using a prediction value being a pixel value of a second block located adjacent to the left side of the target block; a DC prediction mode using a prediction value being an average of the pixel values of the first and second blocks; a diagonal down/left prediction mode using the specified direction being diagonally downward to the left at a 45 degree angle from the horizontal; a diagonal down/right prediction mode using the specified direction being diagonally downward to the right at a 45 degree angle from the horizontal; a vertical right prediction mode using the predicting direction being specified by one pixel rightward and two pixels downward; a horizontal down prediction mode using the predicting direction being specified by two pixels rightward and one pixel downward; a vertical left prediction mode using the predicting direction being specified by one pixel leftward and two pixels downward, a horizontal up prediction mode using the predicting direction being specified by two pixels rightward and one pixel upward; the prediction modes are numbered with increasingly larger numbers, in order of the vertical prediction mode, the horizontal prediction mode, the DC prediction mode, the diagonal down/left prediction mode, the diagonal down/right prediction mode, the vertical right prediction mode, the horizontal down prediction mode, the vertical left prediction mode and the horizontal up prediction mode, and the prediction mode estimating means determines a prediction mode to have the lower mode number among the prediction mode of the first block and the prediction mode of the second block as the estimated prediction mode for the target block