An automatic producing method for a predicted value generation procedure that predicts a value of an encoding target pixel by using a previously-decoded pixel. A parent population is generated by randomly producing predicted value generation procedures each of which is indicated by a tree structure, and a plurality of predicted value generation procedures are selected as parents from the parent population. One or more predicted value generation procedures are generated as children based on a predetermined tree structure developing method which subjects the selected predicted value generation procedures to a development where an existing predicted value generation function can be an end node of a tree. A predicted value generation procedure having a best estimation cost is selected from among the predicted value generation procedures as the parents and children, where the sum of an information content for representing the tree structure and an amount of code evaluated by a predicted pixel value obtained by the tree structure is used as an estimation cost. A final predicted value generation procedure is produced by iterating the relevant operation.