1. A method for estimating a motion vector of a dynamic image, said method comprising the steps of: obtaining the correlation between a coded block occupying an optional area on a screen of a coded current image, and a plurality of candidate blocks which exist in a search area having an optional size on a screen of a reference image temporally different from the current image and which occupies the same area as that of the coded block; selecting a candidate block having the highest correlation from said plurality of candidate blocks to obtain a displacement at that time as a primary motion vector; determining a secondary search area on the basis of said primary motion vector to estimate a positional relationship between a plurality of secondary candidate blocks, which occupy the same area as that of said coded block existing in the determined secondary search area, or said primary motion vector and said reference screen to determine whether said secondary search area or said secondary candidate block is positioned outside of the boundary of said reference screen; correcting the position of said primary motion vector by a correction quantity obtained in the manner that said secondary search area or said secondary candidate block is located inside of the boundary of said reference screen, when it is determined that a part of said secondary search area or secondary candidate block is positioned outside of said boundary; obtaining the correlation between the plurality of corrected secondary candidate blocks, which exist in the corrected secondary search area based on the motion vector, the position of which has been corrected, and which occupy the same area as that of said coded block, and said coded block; selecting a secondary candidate block having the highest correlation from said plurality of secondary candidate blocks to obtain a displacement at that time as a secondary motion vector; and obtaining a desired motion vector on the basis of said primary motion vector, said correction quantity and said secondary motion vector.