1. An image decoding method for decoding a coded stream which includes a plurality of processing units and a header of the processing units, the coded stream being generated by coding a moving picture using inter prediction, the processing units including at least one processing unit divided into a plurality of sub processing units in a hierarchy which decrease in size from a higher level to a lower level, the hierarchy including: a highest hierarchical layer in which a coding unit exists as a largest processing unit; and a lower hierarchical layer which is deeper than the highest hierarchical layer and in which a prediction unit exists as a processing unit smaller than the coding unit, the image decoding method comprising: identifying, by parsing hierarchy depth information stored in the header and indicating a hierarchical layer higher than a lowest hierarchical layer in which a smallest prediction unit exits, a hierarchical layer which is indicated by the hierarchy depth information or a hierarchical layer higher than the indicated hierarchical layer, the hierarchical layer having a prediction unit in which a reference index necessary for decoding is stored; and decoding the prediction unit, using the reference index stored in the prediction unit at the identified hierarchical layer.