1. A motion estimation method for analyzing motion of a target pixel set in a current image with respect to a reference image, each of the current image and the reference image including a plurality of pixel sets, the target pixel set being one of the plurality of pixel sets in the current image, each of the pixel sets including at least one pixel, the motion estimation method comprising the machine-implemented steps of: (A) defining one of the pixels in one of the pixel sets in the reference image as a center of search corresponding to the target pixel set; (B) determining a center error signal based on the target pixel set in the current image and said one of the pixel sets in the reference image that includes the center of search defined in step (A); (C) defining another one of the pixels in the reference image as a target of search with reference to the center of search, a candidate search vector selected from a set of candidate search vectors available for selection, and a step size; (D) determining a target error signal based on the target pixel set in the current image and one of the pixel sets in the reference image that includes the target of search; (E) determining whether or not to update the center of search and the center error signal based on a comparison result between the center error signal and the target error signal; (F) when it is determined in step (E) that the center of search and the center error signal are to be updated, updating the center of search and the center error signal, and updating the set of candidate search vectors available for selection with the step size used in step (C) by removing one of the candidate search vectors that is opposite to the selected candidate search vector from the set if said one of the candidate search vectors that is opposite to the selected candidate search vector is present in the set; (G) repeating steps (C) to (F) by selecting one of the candidate search vectors available for selection from the set and the same step size until there is no candidate search vector available for selection in the set; (H) repeating steps (C) to (G) using a smaller step size until the value of the step size reaches a predetermined value; and (I) computing a motion vector based on the target pixel set in the current image and one of the pixel sets in the reference image that includes the center of search.