1. A moving picture decoding method including a prediction mode, comprising: in said prediction mode, in which prediction mode motion vector information of a current block in a current frame is not transmitted from an encoding side; selecting, from among multiple candidate reference frames, a frame(s) to be referenced to in the prediction mode; determining motion vector information to be used in the prediction mode; and performing prediction processing using the selected reference frame and the determined motion vector information; wherein when candidate blocks within a current frame and adjacent to a current block include blocks that use backward reference frames, a backward reference frame having a smallest index number is selected as the backward reference frame for the current block from the backward reference frames that said blocks using the backward reference frames have.