Abstract: [Object] To provide a speech intelligibility improving apparatus capable of generating highly intelligible speech in various environments without unnecessarily amplifying sound volume. [Solution] A speech intelligibility improving apparatus 250 includes: an envelope surface extracting unit 292 extracting, from a spectrum of speech signal 254 as an object of processing, a curve representing a general outline of peaks of spectral envelope in contact with or along local peaks of spectral envelope of the spectrum; a noise adapting unit 300 modifying spectrum of speech signal 254 based on the curve extracted by envelope surface extracting unit 292; and a sinusoidal wave speech synthesizing unit 305 generating a modified speech signal 260 for the speech improved in intelligibility based on the spectrum modified by noise adapting unit 300.