An image decoding apparatus, method, and medium for decoding a current block included in a bitstream are provided. A first candidate, having a first motion vector that has been used to decode a first block, is searched for from one or more candidates. The first block is spatially or temporally adjacent to the current block. Two or more second candidates, each having a second motion vector being a zero vector, are derived when the first candidate is unavailable in the searching. A current block is decoded using derived candidates. The derived candidates include the two or more second candidates.