In order to make the method of spin trapping technique with 5,5-dimethyl-1-pyrroline-N-oxid (DMPO) more powerful in biological investigation, using electrochemical reaction, we have developed the method to observe an ESR spectrum composed of only a spin adduct of hydrogen atoms (DMPO-H). It has been elucidated that DMPO-H is produced through the hydrogen atom formation, not through the reduction of DMPO.