A method of quantifying an autoantibody includes the steps of reacting a biological sample containing the autoantibody as a test substance with an antigen that is specifically recognized by and bound to the autoantibody, in competition with a known amount of a competition antibody that competes with the autoantibody for binding to the antigen; reacting the autoantibody bound to the antigen with a detection antibody that recognizes and binds to the autoantibody, but does not recognize the competition antibody; measuring a signal derived from the detection antibody bound to the autoantibody; and calculating an amount of the autoantibody contained in the biological sample by utilizing, as an index therefor, an amount of the competition antibody that provides 50% reduction of the signal derived from the detection antibody which is bound to the autoantibody in absence of the competition antibody.