1. An image signal encoding method for encoding an image signal using motion compensation, comprising the steps of: receiving an input image signal for a first picture in a series of pictures; providing a reference image signal of a second picture in the series of pictures, said second picture preceding the first picture; dividing the input image signal into matching blocks, each of the matching blocks including signals for pixels adjacent to each other on a display screen; providing an offset value related to a magnitude of a vector representing a position of each of the blocks in the reference image signal relative to each of the matching blocks in the input image signal; determining a distortion representing differences between signals for pixels in each of the matching blocks in the input image signal, and signals for pixels in the block in the reference image signal which correspond to the signals for pixels in each of the matching blocks in the input image signal; detecting, for each of the matching blocks in the input image signal, a block in the reference image signal which yields a minimum value of an evaluation function which is a sum of said distortion and said offset; and detecting a motion vector representing a position of each of the blocks detected in the reference image signal, relative to each of the matching blocks in the input image signal.