To provide a novel affinity labeling method for conveniently and effectively labeling a target polymer, and a screening method, using the affinity labeling method and screening a target compound or a material (a ligand compound) which interacts with the target polymer.

The ligand compound and the target compound react on a triazine-type dehydration condensing agent. By having the target polymer combine with the target compound by a dehydration condensing reaction, the target polymer reacting on the ligand compound interacting with the target polymer can be labeled effectively.