1. A derivation method of deriving a motion vector for a current block of a picture, the derivation method comprising: deriving one or more first type of candidates for the motion vector of the current block, each of the one or more first type of candidates being derived from a motion vector of a first block; deriving a second type of candidate for the motion vector of the current block when a total number of the one or more first type of candidates is less than a threshold value, the second type of candidate being different from the one or more first type of candidates; and selecting the motion vector of the current block for decoding the current block, from among the one or more first type of candidates and the second type of candidate.