This invention provides a method to detect analyte which comprises preparing fine particles which have a chargeable group in their core and a hydrophilic polymer chain in their shell; obtaining an agglutinated matter by forming a biologically specific bond between a specific residue on the surface of fine particles and analyte, and by simultaneously forming a bond by the electrostatic interaction between impure protein and said particles; and subsequently cleaving only the latter bond by raising ionic intensity. Thus, this invention provides a method to detect analyte with rapidity and high sensitivity with use of agglutination reaction.