The present invention provides an agent for determining Alzheimer's disease, comprising an anti-S38AA antibody, a method of determining Alzheimer's disease in a test animal, comprising detecting an S38AA fragment in a sample collected from said animal, and a method of screening for a substance that treats or prevents Alzheimer's disease, comprising contacting a test substance with a cell permitting measurement of production of a S38AA fragment, measuring the production amount of the S38AA fragment in the cell, and comparing the production amount with that of the S38AA fragment in a control cell free of contact with the test substance, and selecting a test substance that down-regulates the production amount of the S38AA fragment as a substance capable of treating or preventing Alzheimer's disease, based on the comparison results.