1. A speech recognition device comprising: a coefficient storage unit which stores a suppression coefficient representing an amount of noise suppression and an adaptation coefficient representing an amount of adaptation which is generated on the basis of a predetermined noise and is synthesized to a clean acoustic model generated on the basis of a voice which does not include noise, in a manner to relate the suppression coefficient with the adaptation coefficient to each other; a noise estimation unit which estimates noise from an input signal; a noise suppression unit which suppresses a portion of the noise specified by a suppression amount specified on the basis of the suppression coefficient, among from the noise estimated by said noise estimation unit, from the input signal; an acoustic model adaptation unit which generates an adapted acoustic model which is noise-adapted, by synthesizing the noise model, which is generated on the basis of the noise estimated by said noise estimation unit in accordance with an amount of adaptation specified on the basis of the adaptation coefficient, to the clean acoustic model; and a search unit which recognizes voice on the basis of the input suppressed noise by said noise suppression unit and the adapted acoustic model generated by said acoustic model adaptation unit.
4. The speech recognition device according to claim 1 , comprising: a coefficient determination unit which stores the suppression coefficient and the adaptation coefficient stored in said coefficient storage unit, wherein said coefficient determination unit takes a noise estimated by said noise estimation unit as observed data, and updates a suppression coefficient and an adaptation coefficient stored in said coefficient storage unit on the basis of a suppression coefficient and an adaptation coefficient which maximize an expected value of a likelihood function which takes the suppression coefficient and adaptation coefficient of the last update as parameters, when the differences between the suppression coefficient and the adaptation coefficient which are updated last, and the suppression coefficient and the adaptation coefficient which are updated before last are not both equal to or smaller than a predetermined threshold value.