1. A derivation method of deriving a motion vector for a current block of a current picture, the derivation method comprising: deriving, from a first motion vector of a first block that is adjacent to the current block in the current picture, at least one first candidate of a predictor of the motion vector for the current block; deriving, from a second motion vector of a second block that is included in a picture that is different from the current picture, at least one second candidate of the predictor, when a number of the at least one first candidate is less than a maximum number; generating a candidate list that includes the at least one first candidate and the at least one second candidate; and adding, into the candidate list, a zero motion vector as a third candidate of the predictor, when a sum of the number of the at least one first candidate and a number of the at least one second candidate is less than the maximum number, wherein, in the adding, the third candidate is added until a total number of candidates in the candidate list is equal to the maximum number, with at least two third candidates being added.