15. The target speech extraction method of claim 7 , wherein the extracting of the target speech comprises: in a case where the additional information for the target speech is not provided, performing primary speech recognition for the separated sound-source signals by using an HMM (hidden Markov model) as a speech-recognition acoustic model; calculating a closest HMM and a state column thereof for a sequence of the words obtained through the speech recognition; calculating LLR i by using the HMMs; calculating a maximum value by using the calculated LLRi; comparing the maximum value with a predetermined threshold value; and determining the maximum value to be the target speech if the maximum value is larger than the threshold value.
15. The target speech extraction method of claim 7 , wherein the extracting of the target speech comprises: in a case where the additional information for the target speech is not provided, performing primary speech recognition for the separated sound-source signals by using an HMM (hidden Markov model) as a speech-recognition acoustic model; calculating a closest HMM and a state column thereof for a sequence of the words obtained through the speech recognition; calculating LLR i by using the HMMs; calculating a maximum value by using the calculated LLRi; comparing the maximum value with a predetermined threshold value; and determining the maximum value to be the target speech if the maximum value is larger than the threshold value. 16. The target speech extraction method of claim 15 , wherein the LLR i is calculated by using the following Equation: L ⁢ ⁢ L ⁢ ⁢ R i = ⁢ log ⁢ P ⁡ ( X i | W ) P ⁡ ( X i | W _ ) = ⁢ log ⁢ ∏ t ⁢ ⁢ P ⁡ ( x t i | s j m ) ∏ t ⁢ ⁢ P ⁡ ( x t i | s _ j m ) = ⁢ ∑ t ⁢ ⁢ log ⁢ ⁢ P ⁡ ( x t i | s j m ) ⁢ P ⁡ ( x t i | s _ j m ) ≅ ⁢ ∑ t ⁢ ⁢ log ⁢ ⁢ P ⁡ ( x t i | s j m ) - ⁢ log [ 1 K - 1 ⁢ ∑ k = 1 ⁢ ⁢ k ≠ m K ⁢ ⁢ exp ⁡ ( γ ⁢ ⁢ log ⁢ ⁢ P ⁡ [ x t i | s j k ] ) ] 1 / γ ] wherein, w denotes a sequence of the words obtained from the speech recognition unit 130 , w denotes a complement of the w, s j m denotes a complement of the s m j , K is the number of the states included in all the HMMs corresponding to w , γ is a constant term which can be determined experimentally.