IMAGE CODING METHOD AND APPARATUS

To reduce the computation cost of rate/distortion optimization in image compression, the rate/distortion gradient of a frame of interest is classified to m categories. A category information creating unit acquires a threshold λ for a preceding frame as a predictive value λ′ for the frame of interest that is to be coded, and further finely segments a category in which the predictive value λ′ is included. A code amount is then calculated for each of n categories that include the predictive value λ′. A code sequence forming unit selects a category having a target rate, and searches for a threshold between an upper and a lower limit value of the category. A value (S) at the end of the processing is selected as the threshold λ, and a code sequence is formed by using the threshold λ.