To obtain a reagent capable of interacting with nucleic acid or a nucleic acid-related substance and used for analyzing behavior, structure thereof, or the like.

This reactional reagent for a nucleic acid-related substance comprises a polyamino acid derivative containing an amino acid having a reactive functional group composed of hydroxy group, carboxy group or amino group in the side chain besides amino group and carboxy group forming a polypeptide bond as constituent unit and a phenylboronic acid group represented by formula (I) (wherein, R denotes an atomic group formed by reacting with the hydroxy group, carboxy group or amino group which is the reactive functional group in the side chain of the amino acid) and bound through the reactive functional group in the side chain of the amino acid.