It is intended to provide a high-quality aptamer against midkine. An aptamer having an inhibitory activity against midkine； a complex comprising an aptamer having a binding activity or an inhibitory activity against midkine and a functional substance (such as an affinity substance, a labeling substance, an enzyme, a drug delivery medium or a drug)； a pharmaceutical, a cell migration inhibitor, a diagnostic agent and a labeling agent comprising an aptamer having a binding activity or an inhibitory activity against midkine, or a complex containing the aptamer and a functional substance； and the like.
