1. A motion vector predictive encoding method in which a target frame to be encoded is divided into small blocks and a motion-compensating method to be applied to each target small block to be encoded is selectable from among a plurality of motion-compensating methods, wherein: when the motion vector of the target small block is predicted based on the motion vector of an already-encoded small block, if the motion model of the motion vector of the target small block differs from the motion model of the motion vector of the already-encoded small block, then the motion vector of the target small block is predicted by converting the motion vector of the already-encoded small block for the prediction into one suitable for the motion model of the motion vector used in the motion-compensating method of the target small block, and calculating a predicted vector, and a prediction error of the motion vector is encoded.