1. A model adaptation apparatus for adapting a model used in pattern recognition in which input data in the form of a time series is classified into one of a predetermined number of models, said apparatus comprising: data extraction means for extracting input data corresponding to a predetermined model, observed during a predetermined interval, and then outputting the extracted data; and a model adaptation means for adapting said predetermined model using said data extracted during said predetermined interval by means of one of the maximum likelihood method, the mixed statistic method, and the minimum distance-maximum separation theorem; wherein said model adaptation means adapts an acoustic model for ambient noise during a noise observation interval that ends when a speech switch is turned on when a user starts speech.