Provided is a high-quality aptamer against midkine. An aptamer possessing an inhibitory activity against midkine； a complex comprising an aptamer possessing a binding activity or inhibitory activity against midkine and a functional substance (for example, affinity substances, substances for labeling, enzymes, drug delivery vehicles, drugs and the like)； a pharmaceutical drug, cell migration inhibitor, diagnostic reagent, labeling agent and the like comprising an aptamer possessing a binding activity or inhibitory activity against midkine, or a complex comprising the aptamer and a functional substance； a cell migration inhibitor, a diagnostic reagent, a labeling agent and the like.
