The problem to be solved by the present invention is to provide a highly-versatile method for producing a nitroxyl radical derivative, in which position-2 and position-6 in a TEMPO-based compound can be easily substituted, and further, a method for producing a nitroxyl radical derivative, in which a nitrogen nucleus is labeled with 15N. The above-described problem can be solved by reacting a triacetoneamine derivative with ketone or aldehyde in the presence of ammonium salt or a 15N-labeled compound thereof to obtain a 2,6-substituted-4-piperidone derivative.