We study \(μ^+ / μ^−\) separation using decay electron information in a cosmic ray stopping muon sample in order to separate atmospheric \(ν / {\bar ν}\). We discuss event selection method defined for cosmic ray MC and data sample to identify decay electrons and apply it to atmospheric neutrino MC. Adding this selection, we obtained average muon decay time by fitting the muon decay time distribution. These results are consistent with the previously reported values. Therefore, we confirmed that this selection worked well. In future prospect, we will make distributions of the difference of log likelihood in atmospheric neutrino MC using presented results in order to develop event-by-event separation.