To make more linear a detection line obtained from RF concentration and a latex coagulation degree in a rheumatic factor quantifying method for causing a latex coagulation reaction by mixing a sample and latex particles sensing human γ-globulin, measuring an optical intensity and quantifying rheumatic factor in the sample.

Mixed liquid is made by mixing a sample and latex particles sensing human γ-globulin, a latex coagulation reaction is produced by causing at least one kind of a water soluble compound selected from the group of tri- alkyl amine, its salt and fourth class ammonium salt and polyvinyl pyrrolidone or dextran to coexist in the mixed liquid, an optical intensity is measured and a rheumatic factor in the sample is quantified based on this measured value.