1. A motion estimator, employing a three-step hierarchical search block-matching algorithm for obtaining a motion vector by block-matching between a current block and its corresponding block, comprising: a memory block for storing a candidate block corresponding to the current block; a matching unit for obtaining a motion vector by matching said current block with the corresponding candidate block of said corresponding candidate; and a control unit for supplying candidate blocks in the memory blocks to said matching unit for block-matching according to a predetermined sequential order, and writing said candidate current blocks to the addresses no longer used in said memory blocks at the same time Step 3 in the three-step hierarchical search block-matching algorithm is being performed.