The present invention provides a hearing aid in which at least one portion of an input sound signal is divided into a frequency band signal, and a single or a plurality of the frequency band signals are subjected to noise to generate a Noise-Vocoded Speech Sound signal, which a user can hear. Such a hearing aid facilitates activation of the brain and is expected to provide an effect on treatment or training of people with a neural disorder. Such a hearing aid lets a Noise-Vocoded Speech Sound signal recognized by utilizing normal portions of the brain to the maximum level and the Noise-Vocoded Speech Sound signal is compensated for by other normal portions of the brain, in order to let a person with hearing difficulty understand the meaning of the input sound.