7. The apparatus of claim 6 , wherein the histogram normalization unit comprises: a matching unit matching a noisy backward cumulative distribution function of the speech signal in a noisy environment and a clean backward cumulative distribution function of the speech signal in a clean environment; an estimation unit estimating a value of the backward cumulative distribution function by estimating a value of the noisy background cumulative distribution function of the speech signal in the noisy environment to be a value of the clean backward cumulative distribution function of the speech signal in the clean environment; and a normalization unit normalizing the histogram using the estimated value.
7. The apparatus of claim 6 , wherein the histogram normalization unit comprises: a matching unit matching a noisy backward cumulative distribution function of the speech signal in a noisy environment and a clean backward cumulative distribution function of the speech signal in a clean environment; an estimation unit estimating a value of the backward cumulative distribution function by estimating a value of the noisy background cumulative distribution function of the speech signal in the noisy environment to be a value of the clean backward cumulative distribution function of the speech signal in the clean environment; and a normalization unit normalizing the histogram using the estimated value. 8. The apparatus of claim 7 , wherein the normalization unit calculates a probability distribution function from a feature vector of the estimated value, calculates a backward cumulative distribution function by cumulating the probability distribution function in an order from a greatest to a smallest value, and normalizes the histogram utilizing the backward cumulative distribution function.