[Problem] To identify an enzyme that decomposes active reelin into an inactive form and to provide an effective treatment and research means for diseases such as Alzheimer's disease based on active reelin decrease using this enzyme and/or a gene thereof. [Solution] ADAMTS-3, which is used to decompose active reelin into inactive reelin, and functional derivatives thereof. A method for using ADAMTS-3 that uses ADAMTS-3 or a functional derivative thereof for treatment or research relating to the physiologic functions of reelin or the promotion or suppression thereof.