1. An adaptable shape image encoding apparatus for cutting out partial images from a base image having a predetermined shape so that the partial images have a size smaller than that of the base image comprising: a reference block description table that stores predetermined rules for selecting reference blocks used to determine a motion vector, the reference blocks being blocks adjacent to a block of the base image; preprocessing means for generating preprocessing data by generating a plurality of motion vectors for the block of the base image and orthogonal transformation coefficients respectively corresponding to the plurality of motion vectors, the respective motion vectors being generated based on a correlation between the block of the base image and reference blocks selected according to the predetermined rules, wherein each of the predetermined rules for selecting reference blocks stored in the reference block description table defines different reference blocks adjacent to the block of the base image that are selected and used to generate a different motion vector of the plurality of motion vectors generated for the block of the base image; storing means for storing the preprocessing data; and partial image encoding means for obtaining preprocessing data corresponding to a block included in a partial image cut out from the base image and encoding the partial image using the preprocessing data.