The present invention relates to (1) A reagent for an immunoassay of a target substance existing in a free form and a bound form in a specimen, comprising a latex 1 which is immobilized with a monoclonal antibody 1 for the target substance, and a latex 2 which has a different mean particle size from the latex 1 and is immobilized with a monoclonal antibody 2 having a different recognition site for the target substance from the antibody 1, (2) An immunoassay method comprising reacting the target substance with the reagent of (1) and determining an amount of the substance based on the result of an agglutination reaction among the target substance, the latex1 and the latex2, and (3) A reagent kit comprising a reagent of (1) and a reagent containing an agglutination accelerator for an antigen-antibody reaction.