1. A noise segment/speech segment determination apparatus which determines whether an input signal segment is a noise segment or a speech segment, the apparatus comprising: an analog-to-digital conversion unit for converting a speech signal having ambient noise superimposed thereon into a digital signal; a data extraction unit for extracting the digital signal as segment data having a predetermined duration; an autocorrelation function computation unit for computing an autocorrelation function of the extracted data &lsqb;provided that an analysis order is taken up to a “p-order,” R(0), R(1), R(2),... R(p)&rsqb;; an autocorrelation function normalizing unit for obtaining a normalized autocorrelation function by means of dividing the autocorrelation function by R(0); a normalized autocorrelation function count unit for counting the number of times normalized autocorrelation functions have arisen; a normalized autocorrelation function storage unit for storing the normalized autocorrelation functions as normalized autocorrelation function vectors &lsqb;(r(1), r(2),... r(p))); a noise vector region/speech vector region/undefined vector computation unit which classifies and computes a plurality of normalized autocorrelation function vectors into one or a plurality of noise vector regions, one or a plurality of speech vector regions, and undefined vectors, when the number of normalized autocorrelation function vectors stored in the normalized autocorrelation function storage unit has reached a predetermined number; a noise vector region/speech vector region/undefined vector storage unit for storing the noise vector region, the speech vector region, and undefined vectors; and a normalized autocorrelation function vector determination unit which determines to which, if any, of a plurality of noise vector regions the latest normalized autocorrelation function vector stored in the normalized autocorrelation function storage unit pertains, and which determines the acquired signal segment as corresponding to a noise section when the vector pertains to one of the plurality of noise vector regions and determines the acquired signal segment as corresponding to a speech section when the vector does not pertain to any of the plurality of noise vector regions.