1. A motion estimation method using a plurality of reference frames, the method comprising: analyzing a plurality of reference indexes of a current block, the plurality of reference indexes indicating reference frames of neighboring blocks that have already been coded and a plurality of reference indexes of a block corresponding to the current block and its neighboring blocks in a previous frame; performing a first linear combination of a number of first blocks that have a same reference index among the neighboring blocks of the current block, and a number of second blocks that have the same reference index among the block corresponding to the current block and its neighboring blocks in the previous frame; estimating a norm of a motion vector of the current block based on a norm of an average motion vector of the neighboring blocks of the current block; calculating a posterior probability of the estimated norm of the motion vector of the current block with respect to each of the plurality of reference frames using the estimated norm of the motion vector of the current block as a feature vector; performing a second linear combination of a value of the first linear combination and the posterior probability of the estimated norm of the motion vector of the current block with respect to each reference frame and performing motion estimation on the current block sequentially using the plurality of reference frames according to values of the second linear combination; and calculating a probability distribution of the reference frame index using an equation: P ⁡ ( i X ; X ) = P X ⁡ ( i X ) = e - a ⁡ ( i X + 1 ) ∑ j = 0 r - 1 ⁢ e - a ⁡ ( j + 1 ) ⁢ ∑ k = 0 r - 1 ⁢ P X ⁡ ( i x ) = ∑ k = 0 r - 1 ⁢ P X ⁡ ( i X = k ) ⁢ P ^ k ⁡ ( N ) + ∑ k = 0 r - 1 ⁢ P X ⁡ ( i X = k ) ⁢ P ^ k C ⁡ ( N ) = Γ N ⁢ Γ N C , ⁢ for i X ∈ R , R = { 0 , … ⁢ , r - 1 } , a > 0 , .