1. An image coding method for generating a coded stream which includes a plurality of processing units and a header of the processing units, 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 coding method comprising: coding the moving picture using inter prediction; writing, into the header, hierarchy depth information which is information for identifying a hierarchical layer having a prediction unit in which a reference index necessary for decoding is stored and indicates a hierarchical layer higher than a lowest hierarchical layer in which a smallest prediction unit exists; and writing, into a prediction unit located at a hierarchical layer indicated by the hierarchy depth information or located higher than the indicated hierarchical layer, the reference index to be applied to the prediction unit.