1. A moving picture decoding method, carried out by computing system, including a prediction mode, in which prediction mode motion vector information of a current block in a current frame is not transmitted from an encoding side, comprising: in said prediction mode: selecting one reference frame from among a plurality of candidate reference frames including: a reference frame referenced by one or a plurality of adjacent blocks within a current frame and adjacent to a current block; and a prescribed reference frame not based on the reference frame referenced by the adjacent blocks, as the reference frame to be used in the prediction mode; determining a motion vector to be used in the prediction mode; and performing prediction processing using the selected reference frame and the determined motion vector.