1. A hearing aid comprising: a first band filtering portion including a band filter which extracts a signal with a predetermined frequency band when a sound signal is input to the hearing aid; an envelope extracting portion including an envelope extractor which extracts an envelope of the frequency band signal extracted by the band filter; a noise source generating a noise for deteriorating the input sound signal; a second band filtering portion including a band filter which extracts a noise signal corresponding to the predetermined frequency band from the noise generated by the noise source; and a multiplying portion which subjects each said sound signal with a predetermined frequency band to said noise signal corresponding to the predetermined frequency band by multiplying an output from the envelope extracting portion by an output from the second band filtering portion, and configures a Noise-Vocoded Speech Sound signal therefrom; wherein the Noise-Vocoded Speech Sound signal is outputted by the hearing aid.