1. An image decoding method for decoding a coded stream including encoded picture, comprising: a first step for determining a motion vector of a target block to be decoded, by selecting a motion vector among motion vectors of candidate blocks including adjacent blocks adjacent to the target block to be decoded, according to coding mode information obtained from the coded stream; and a second step for generating a prediction image by using the motion vector of the target block to be decoded and a reference picture, wherein, in a motion vector selection process of the first step, candidate blocks are selected among first blocks adjacent to the target block to be decoded in the same frame as the target block to be decoded and a second block located at the same position of the target block to be decoded in a different frame from the target block to be decoded, and wherein, in the motion vector selection process of the first step: excluding a block located on the upper side of the target block to be decoded in the first blocks from the candidate blocks in the case that the block located on the upper side of the target block to be decoded has the same motion vector as a block located on the left side of the target block in the first blocks, excluding a non-referenceable block in the first blocks from the candidate blocks in the case that the first blocks have a non-referenceable block, and adding another block or blocks to the candidate blocks, wherein the number of blocks to be added is changed based on the number of blocks that have been selected as candidate blocks.