1. A motion vector predictive encoding method in a moving picture encoding scheme in which a picture to be encoded is divided into blocks and encoding is performed using motion compensation for each block, the method comprising: a step of performing a motion search for a block to be encoded in the picture to be encoded using an encoded reference picture to calculate a motion vector; a step of setting a plurality of blocks which include at least one of encoded blocks in the picture to be encoded and encoded blocks in a encoded picture and in predetermined positions relative to the position of the block to be encoded in the picture to be encoded as primary candidate blocks, and determining N (N is an integer greater than or equal to 2) primary candidate reference motion vectors from motion vectors used in encoding the primary candidate blocks; a step of calculating degrees of reliability of the primary candidate reference motion vectors which quantitatively represent effectiveness in motion vector prediction of the block to be encoded, using encoded picture information for each of the primary candidate reference motion vectors; a step of selecting top M (M is a predetermined integer greater than or equal to 1 and less than N) primary candidate reference motion vectors with higher degrees of reliability from the N primary candidate reference motion vectors as secondary candidate reference motion vectors; and a step of calculating a predictive motion vector of the block to be encoded using the secondary candidate reference motion vectors, and encoding a residual between the motion vector obtained through the motion search of the block to be encoded and the predictive motion vector as encoded information of the motion vector.