Provided are an agglutination-inhibition assay and a reagent for agglutination-inhibition assay, which can be used for measuring a ligand in a sample at high sensitivity in a wide range from the low-concentration range to the high-concentration range and have good reproducibility of measurement. Specifically, provided are an agglutination-inhibition assay and a reagent for agglutination-inhibition assay, in which used are an insoluble carrier particle carrying a ligand, a specific receptor in the free-form and an insoluble carrier particle carrying a specific receptor which binds to a different site on the ligand than the receptor in the free-form.