1. A method of dividing a frame of a target image whose motion is to be detected into plural target blocks and detecting motion vectors representing motions of said target blocks relative to a reference frame of image, said method comprising the steps of: dividing a target block into a plurality of blocks each of which has different pixel positions from each other wherein each of the plurality of blocks is allocated for one of vectors representing motions of said target block; comparing said plurality of blocks with a reference image block extracted from a search area within said reference frame of image, with respect to pixel data; finding degrees of similarity of said plurality of blocks to said reference image block; selecting one of said plurality of blocks according to the found degrees of similarity; and detecting a vector allocated to the selected block as a candidate of the motion vector representing motion of the target block.