an apparatus for detecting an interaction between a biopolymer and a ligand characterized by consisting of a block provided with a reaction system having a reaction filed in which the biopolymer is immobilized , a supplying channel connected to the reaction field for supplying a sample to the reaction field and a recovery channel for recovering the sample having passed at least a part of the reaction field ; and a means of measuring the flow potential of the immobilized biopolymer while supplying the sample to the reaction field via the supplying channel and recovering the sample having passed at least a part of the reaction field via the recovery channel . by using this apparatus , the interaction between a large number of biopolymers and ligands can be detected simultaneously .