A hearing aid gain determination system includes: a biological signal measurement section for measuring an electroencephalogram signal a sound group determination section for determining a frequency of a sound group including first, second and third sounds a sound pressure determination section for determining sound pressures of the first to third sounds so that the respective sound pressures are less than a threshold and that the first to third sounds consecutively decrease an output section for presenting the first to third sounds at the determined frequency and sound pressures a characteristic amount extraction section for extracting a characteristic amount concerning time-frequency information of an event-related potential in a time range after presenting the first to third sounds a gain determination section for determining a hearing aid gain for the frequency of the sound stimulation group against a predetermined criterion based on the characteristic amounts.
