A novel carboxypeptidase and the encoding gene thereof were successfully identified by screening a human hippocampus extract using brain-APP-cleaving activity as an index. This protein and its gene are useful in, for example, preventing, treating, examining, and diagnosing diseases that cause accumulation of Aβ in the brain, including senile dementia, Alzheimer's disease, Down's syndrome, hereditary cerebral hemorrhage, and cephalic contusion.