A class synthesizing circuit classifies an aimed-at data item into one of a plurality of classes specified in advance, according to a plurality of data items disposed around the aimed-at data item. A coefficient holding and class-code selection circuit stores conversion information for the aimed-at data item, for each class. An estimation-prediction calculation circuit converts the aimed-at data item to a higher-quality data item according to the conversion information. The class synthesizing circuit classifies the aimed-at data item into a different class according to whether the aimed-at data item is missing.