To make it possible to apply to various biochemical auto-analyzers, to quickly and easily measure a lot of specimens, and to hardly generate a determination failure caused by nonspecific agglutination.

Agglutination by an antigen-antibody reaction between an antiphospholipid antibody and an antigen is measured based on a change of absorbance, using this antiphospholipid antibody measuring reagent. The antiphospholipid antibody measuring reagent comprises an antigen reagent containing an insoluble carrier adsorbed with a lipid antigen comprising cardiolipin, lecithin and cholesterol, and containing a water-soluble polymer for blocking the lipid antigen adsorbed onto the insoluble carrier, and a buffer solution for dilution containing the water-soluble polymer.