An object of the present invention is to provide an immunoassay of PSA using an agglutination accelerator, which has an agglutination accelerating effect equal to or stronger than the known agglutination accelerator; hardly generates non-specific turbidity; and hardly generates salting out even in a solution with a high salt concentration. The present invention relates to an immunoassay of a prostate-specific antigen comprising performing an antigen-antibody reaction in the presence of a polymer having a monomer unit derived from a monomer represented by the following general formula [2]: ##STR00001## (wherein R.sup.1-R.sup.3 are each independently a hydrogen atom or an alkyl group optionally having a hydroxyl group; R.sup.4 is an alkylene group; R.sup.5 is an alkylene group optionally having a substituent and optionally having an oxygen atom in a chain; R.sup.6 is a hydrogen atom or a methyl group, and X is an oxygen atom or a --NH-- group), and a kit of reagent for an immunoassay comprising a reagent containing an agglutination accelerator for the immunoassay.