The present invention provides a simple and efficient method for preparing safer compounds which prevent and ameliorate conditions and diseases caused by reduction in brain function and are suitable for use in medicines and food products. Specifically, the present invention provides: a method for preparing peptides each consisting of an amino acid sequence shown in any of SEQ ID NOs: 1-4 or 16, said method being characterized in that mammalian milk or milk protein is fermented using at least one lactic acid bacterium and/or a processed product thereof, and then the peptides are collected from the resultant fermented milk.
