1. A speech recognition apparatus comprising: at least one computer processing device implementing a noise estimation unit configured to estimate a noise spectrum included in an input speech signal; a noise subtraction unit configured to subtract the noise spectrum from the input speech signal and output a noise subtracted signal; a speech feature extraction unit configured to: (i) calculate a first vector that represents a frequency spectrum of the input speech signal, (ii) calculate a second vector that represents a frequency spectrum of the noise subtracted signal, (iii) combine the first vector and the second vector to obtain a combined vector, and (iv) calculate a feature vector by compressing the combined vector into a predetermined dimension; and a speech recognition unit configured to perform speech recognition by performing a pattern matching of the feature vector with a standard speech pattern obtained beforehand.