Problem to be solved: to provide a method of labeling radioactive elements.A method of labeling a radioactive element comprising the steps of providing a Diaminophenyl compound represented by the formula (I) and a biomolecule, etc., and the step of reacting the above described Diaminophenyl compound with the compound labeled with the following formula (II) and at the normal temperature.Diagram【課題】放射性元素の標識方法等の提供。【解決手段】式(I)で表され、生体分子等が結合されたジアミノフェニル化合物を提供する段階と、上記ジアミノフェニル化合物と、下記式(II)で表され、放射性元素が標識された化合物とを常温で反応させる段階とを含む放射性元素の標識方法。【選択図】図3