The speech sound intelligibility assessment system includes: an output section for presenting a speech sound to a user; a biological signal measurement section for measuring an electroencephalogram signal of the user; a positive component determination section for determining presence/absence of a positive component of an event-related potential in the electroencephalogram signal in a zone from 600 ms to 800 ms from a starting point, which is a point in time at which the output section presents a speech sound; a negative component determination section for determining presence/absence of a negative component of an event-related potential in the electroencephalogram signal in a zone from 100 ms to 300 ms from the same starting point; and an assessment section for evaluating whether the user has clearly aurally comprehended the presented speech sound or not based on the results of determination of presence/absence of the positive and negative components, respectively.