An object of the present invention is to provide a peptide having an excellent effect in terms of preventing deterioration in cognitive function and improving cognitive function, and provide its usage. The present invention uses the tripeptide represented by Tyr-Leu-Gly as an active ingredient of a composition for improving cognitive function or a composition for promoting brain-derived neurotrophic factor expression in the hippocampus. These compositions include a food composition, a food additive composition, and a pharmaceutical composition.