Disclosed are: an agent, an apparatus and a method for the diagnosis of a disease associated with the abnormality in γ-secretase a method for providing information for the diagnosis of a disease associated with the abnormality in γ-secretase a method for monitoring the condition or the degree of progression of a disease associated with the abnormality in γ-secretase and a method for determining the therapeutic effect of a therapeutic agent on a disease associated with the abnormality in γ-secretase. More specifically disclosed are: a diagnostic agent for a disease associated with the abnormality in γ-secretase, which comprises an antibody capable of recognizing a digestion product of an alcadein with α-secretase or γ-secretase or a fragment of the antibody and others.
