As a technique enabling to simply and accurately diagnose human T cell leukemia virus type 1 (HTLV-1) related disease, there is provided a diagnostic method for an HTLV-1 related disease, comprising the steps of: measuring the amount of a marker protein in a blood sample taken from a subject, wherein the marker protein is at least one selected from the proteins listed in Tables 1 and 2; and, if a measured value is a predetermined reference value or more or the predetermined reference value or less, determining that the subject suffers from the HTLV-1 related disease or determining that the subject is in remission of the HTLV-1 related disease.