The invention provides a motion vector conversion method by which the coding efficiency in image coding of MPEG4 in an image information conversion method can be augmented. In the motion vector conversion method for an image information conversion method wherein a bit stream representative of interlaced scanned image compression information of MPEG2 is inputted and a bit stream representative of progressively scanned image compression information of MPEG4 is outputted, 16.times.16 motion vectors of MPEG2 of the inputted bit stream representative of image compression information of MPEG2 are accepted successively, and 8.times.8 motion vectors of MPEG4 and 16.times.16 motion vectors of MPEG4 are produced successively based on the 16.times.16 motion vectors of MPEG2. Every other one of I frames and P frames of the bit stream of MPEG2 is dropped to produce a bit stream of MPEG4 of a reduced frame rate and a low bit rate.