A speech sound intelligibility assessment system includes: a biological signal measurement section for measuring an electroencephalogram signal of a user; a presented-speech sound control section for determining a speech sound to be presented by referring to a speech sound database retaining a plurality of monosyllabic speech sounds; an audio output section for presenting the speech sound determined by the presented-speech sound control section as an audio; a characteristic component detection section for utilizing the electroencephalogram signal of the user measured by the biological signal measurement section to determine presence or absence of a characteristic component of an event-related potential at 800 ms±100 ms from a point of presenting the audio; and a speech sound intelligibility assessment section for, based on a result of determination by the characteristic component detection section, determining whether the user has aurally comprehended the speech sound or not.