1. A motion estimation system comprising: an initializing module configured to generate first and second initial motion fields by allocating an initial motion vector to each block of a first motion field related to a first motion change of an image which accompanies a change from an (n−1)-th frame to an n-th frame and to each block of a second motion field related to a second motion change of the image which accompanies a change from the n-th frame to the (n−1)-th frame; and a candidate test module configured to generate first and second random motion fields by comparing the similarity function value of the initial motion vector allocated to each block of each of the first and second initial motion fields with the similarity function value of a random motion vector, and configured to generate first and second spatial propagation motion fields by comparing the similarity function value of a motion vector allocated to each block of each of the first and second random motion fields with similarity function values of motion vectors allocated to adjacent blocks, and configured to generate first and second optimum motion fields by comparing the similarity function value of a motion vector allocated to each block of the first spatial propagation motion field with the similarity function value of a motion vector allocated to each block of the second spatial propagation motion field.