An immunoassay method comprising: forming, on a solid phase, a complex comprising a measurement substance in a sample, an alkaline phosphatase (ALP) labeling substance capable of binding to the measurement substance, and a first binding substance which is capable of binding to the measurement substance by antigen-antibody reaction and is capable of binding to the solid phase; reacting ALP in the ALP-labeling substance bound to the measurement substance in the complex, with a substrate for ALP contained in a substrate solution, in the presence of an auxiliary reagent solution comprising an inhibitor for inhibiting an endogenous ALP; and detecting a reaction product produced by the reaction between ALP and the substrate.