A pH-responsive polymer comprising a biocompatible polymer to which a group which is electrically neutral under a pH environment exceeding 7 and is cationically changed at pH 7 or less is bonded.7を超えるpH環境下で電気的に中性でありpH7以下でカチオン性に変化する基が結合した生体適合性ポリマーからなる、pH応答性ポリマー。