A decoder, comprising: circuitry; and memory, wherein the circuitry is configured to, using the memory, in inter prediction processing: derive a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed; derive a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector; and generate a prediction image of the first current block by performing motion compensation using the second motion vector, wherein a mode of the inter prediction processing is a merge mode, characterized in that the circuitry, in the inter prediction processing, is configured to derive a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a picture including the first current block, and derive the third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a picture different from the picture including the first current block, derive a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and generate a prediction image of the second current block by performing motion compensation using the fourth motion vector. circuitry; and memory, wherein the circuitry is configured to, using the memory, in inter prediction processing: derive a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed; derive a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector; and generate a prediction image of the first current block by performing motion compensation using the second motion vector, wherein a mode of the inter prediction processing is a merge mode, characterized in that the circuitry, in the inter prediction processing, is configured to derive a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a picture including the first current block, and derive the third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a picture different from the picture including the first current block, derive a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and generate a prediction image of the second current block by performing motion compensation using the fourth motion vector. derive a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed; derive a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector; and generate a prediction image of the first current block by performing motion compensation using the second motion vector, wherein a mode of the inter prediction processing is a merge mode, characterized in that the circuitry, in the inter prediction processing, is configured to derive a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a picture including the first current block, and derive the third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a picture different from the picture including the first current block, derive a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and generate a prediction image of the second current block by performing motion compensation using the fourth motion vector.