1. A speech discriminability assessment system, comprising: a biological signal measurement processor configured to measure an electroencephalogram signal of a user; a presented-speech sound control processor configured to determine a speech sound to be presented to the user by referring to a speech sound database retaining a plurality of monosyllabic sound data; an audio presentation processor configured to present an audio associated with the determined speech sound to the user; a character presentation processor configured to present a character associated with the determined speech sound to the user, subsequent to the presentation of the audio by the audio presentation processor; an unexpectedness detection processor configured to detect presence or absence of an unexpectedness signal from the measured electroencephalogram signal of the user, the unexpectedness signal representing a positive component at 600 ms±100 ms after a time point when the character was presented to the user; and a speech sound discriminability determination processor configured to determine a speech sound discriminability based on a result of detection by the unexpectedness detection processor.