and a coding field have the same parity a second chrominance motion vector generation unit is selected by the selection unit when a reference field and a coding field are a top field and a bottom field respectively a third chrominance motion vector generation unit is selected by the selection unit when a reference field and a coding field are a bottom field and a top field respectively a method for calculating a chrominance motion vector parallel to a luminance motion vector depends on the coding field parity and reference field parity of a luminance motion vector the calculation method differs in the following three case a case where the coding field parity and reference field parity are the same a case where the coding field and reference field are top and bottom fields respectively and a case where the coding field and reference field are bottom and top fields respectively therefore in the present invention an optimal one is selected from the three types of chrominance motion vector generation units calculating a chrominance motion vector parallel to a luminance motion vector depending on the coding field and the reference field and a chrominance motion vector is generated specifically if the reference field parity and coding field parity are the same the first chrominance motion vector generation unit calculates a chrominance motion vector as follows assuming that a luminance motion vector indicating the vertical displacement of one luminance pixel of a field image by the value u201c 1 u201d of the vector component as units and a chrominance motion vector indicating the vertical displacement of one chrominance pixel of a field image by the value u201c 1 u201d of the vector component as units are mvy and mvcy.