Provided is a method for the simple and highly accurate assay of PSA by a one-step reaction that does not use carriers having different average grain sizes. Also provided is a reagent therefor. The PSA assay method comprises sensitizing insoluble carriers having the same average grain size within a range of greater than 0.20 μm but 0.40 μm or less using two types of anti-PSA monoclonal antibodies having different epitopes that are anti-PSA monoclonal antibodies that will react with both free PSA and PSA-ACT, which is a complex of free PSA and α1-antichymotrypsin, and bringing the carriers into contact with a sample in the presence of an aggregation promoter.
