An image decoding method decodes a coded stream which includes processing units and a header of the processing units, and which is generated by coding a moving picture using inter prediction. The processing units includes at least one processing unit divided in a hierarchy, the hierarchy including a highest hierarchical layer in which a coding unit exists as a largest processing unit and a lower hierarchical layer in which a prediction unit exists. The method includes 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 includes a prediction unit in which a reference index is stored. The prediction unit is decoded using the reference index.