Recent studies on nivolumab, an anti-programmed death 1 antibody, demonstrate dramatic improved outcomes for patients with non-small-cell lung cancer (NSCLC). However, serious immune-related adverse events (irAEs) can occur because of nivolumab. The efficacy of the pretreatment neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR) as predictive factors for serious irAEs was examined in a retrospective review of 37 outpatients with NSCLC treated with nivolumab at Osaka-fu Saiseikai Noe hospital from December 2015 to June 2018. NLR and PLR were calculated by dividing the number of neutrophils and platelets by that of lymphocytes measured in the peripheral blood. No risk factors in patient characteristics or laboratory data were seen in patients with or without irAEs. ROC curve analysis found the cut-off values of NLR and PLR were 2.95 and 191, respectively, and AUC of NLR and PLR were 0.61 and 0.68, respectively. The irAE incidence at NLR ≧ 2.95 or PLR ≧ 191 was higher than those at NLR < 2.95 or PLR < 191. The incidence of irAEs at NLR ≧ 2.95 and/or PLR ≧ 191 was also higher than that at NLR < 2.95 and PLR < 191. All Grade 3 or higher irAEs were detected at NLR ≧ 2.95 and/or PLR ≧ 191. The combined biomarker, NLR and PLR, is suggested to be useful as a predictive marker for the occurrence and severity of irAEs in patients with NSCLC.