The present disclosure relates to a method for diagnosing or providing information for diagnosing Alzheimer's disease or a brain disease, and a composition and a diagnostic kit for diagnosing Alzheimer's disease or a brain disease using miR-485-3p. The present disclosure enables objective data analysis of the diagnosis of Alzheimer's disease or a brain disease by measuring the expression level of miR-485-3p in blood, minimizes risk to a patient by measuring the concentration of amyloid beta 42 in saliva, and enables quick and accurate diagnosis. Accordingly, the present disclosure is very useful for preventing Alzheimer's disease or a brain disease.