The present invention addresses the problem of providing a method whereby disease activity of rheumatoid arthritis is evaluated reliably and with good sensitivity by a simple method, and a kit for use in the method. The present invention furthermore provides a method for accurate diagnosis of polymyalgia rheumatica and relapsing polychondritis, which are similar rheumatic diseases, a method for differentiating rheumatoid arthritis, and a kit for use in the same. The present invention provides a Jacalin-binding O-linked oligosaccharide epitope as a negative marker for diagnosing rheumatoid arthritis, and enables more accurate assessment of disease condition by more objectively determining disease activity of rheumatoid arthritis using the amount of variation of a value obtained by multiplying the amount of MMP-3 in the blood or the amount of bound MMP-3 and ABA, ACA, or ACG in the blood by the reciprocal of the amount of bound MMP-3 and Jacalin in the blood. Through the discovery that an LEL or STL-reactive su
