The present invention provides a method of measuring the activity of neprilysin, etc. More specifically, the present invention provides a method of measuring the activity of neprilysin in nerve cells; a method of screening a protein, a peptide or a compound enhancing the activity or expression of neprilysin nerve cells by measuring the activity of neprilysin; a method of enhancing the activity or expression of neprilysin; and so on. Thus, the compound enhancing the activity and/or expression of neprilysin, which is obtained by the screening method characterized by using the method of measuring the activity of neprilysin in accordance with the present invention, is useful as a preventive and/or therapeutic agent for Alzheimer's disease. The method of measuring the activity of the present invention can be used for presymptoms diagnosis of Alzheimer's disease.