1. In a motion vector detection apparatus for, in order to obtain a motion vector between a reference image and a current original image, matching and searching, with a macroblock (MB) currently processed in the original image, a search region of the reference image which region is set around a position corresponding to the macroblock (MB), a method of determining the search region comprising: determining, using a computer having a processor, a search unit with a size and a ratio of height to width such that the search region made up of an aggregation of the search units suitable for the configuration of the search region can be configured; obtaining a statistics amount of obtained motion vectors in a vector information readout region of the original image set around a position of the MB being processed or in a vector information readout region of the reference image set around a position corresponding to the MB being processed; analyzing the statistics amount with the processor, thereby predicting an appearance tendency of a motion vector; and deciding with the processor, a combination of search units so as to form an expanded search region expanded in a direction to increase an occurrence probability of a motion vector for the MB being processed based on the analysis, wherein: the obtaining includes calculating with the processor, a vector mean value and a vector dispersion value of the obtained motion vectors in the vector information readout region; and the analyzing includes performing an analysis for the vector mean value and the vector dispersion value based on a predetermined mean value threshold and a dispersion value threshold.