To accurately measure the disease activity of rheumatic disease by sampling blood in a measurement receptacle to allow it to react with a material producing a physiologically active substance in the blood by contacting it with the blood, and measuring a quantity of the physiologic active substance.

Endotoxin is preferable due to high induction activity as a material inducing production of a physiologically active substance, and concentration of the endotoxin in the whole blood in the case of contact with blood is set at about 0.6-100,000 EU. The producing physiologically active substances are a tumor necrosis factor α, a tumor necrosis factor β, and interleukin. Endotoxin free water in the same quantity as sampled blood amount is obtained in a measurement receptacle, and stirred for about an hour at about 37°C and extracted to set the content of endotoxin in the extracted liquid to about 0.5 EU/ml or less. Induction of cytokine is caused conspicuously when the content of endotoxin in the receptacle exceeds 0.5 EU/ml. Quantity of physiologically active substance produced in the blood is determined by oxygen immunoassay and then state of rheumatoid disease is judged thereby.