1. A motion picture recording method which performs coding processing of an input picture through inter prediction to record a motion picture, the method comprising: performing a block-based motion search for the input picture to calculate a motion vector of each block; setting up blocks having a similar motion vector as a similar motion area, by comparing the motion vector of a previously coded block with the motion vector of a block to be subjected to coding, the similar motion vector included in each motion vector calculated in the block-based motion search; calculating a representative motion vector by use of a motion vector provided by each block included in the similar motion area set up in the similar motion area; selecting either the representative motion vector or the motion vector provided by each block, so as to reduce the amount of generated codes regarding the motion vector, as a selected motion vector which is to be used for each block in the similar motion area; generating encoded data by use of the selected motion vector and a prediction picture of each block generated using the selected motion vector, for each block in the similar motion area; and calculating, as a block-based cost calculation, a coding cost for each block when the motion vector calculated in the block-based motion search step is used; and calculating, as a similar motion area cost calculation, a coding cost for each similar motion area when the representative motion vector is used; wherein, based on the result of a comparison of a sum total of coding costs of the plurality of blocks calculated in the block-based cost calculation with the coding cost of the similar motion area calculated in the similar motion area cost calculation, for a plurality of blocks included in the similar motion area, the selecting determines which of the respective motion vector calculated in the representative motion vector calculation and the motion vector provided by each block is to be used, for each block in the similar motion area.