A 64-year-old man with advanced gastric cancer presented with chief complaints of chest pain. His preoperative blood examination revealed positive results for serum HIV-antibody. His HIV-RNA level was 1.0×10 / 5 copies/mL, and his CD4lymphocyte count was 491 cell/mL; the patient was diagnosed with advanced gastric cancer and HIV infection. Distal gastrectomy with D2 lymphadenectomy and Roux-en-Y reconstruction were performed for treatment of the gastric cancer. Pathological examination revealed T3(SS)N3aM0, Stage III C cancer. After surgery, the patient was administered S-1 monotherapy as adjuvant treatment with antiretroviral therapy including tenofovir/emtricitabine and raltegravir. He completed 8 courses of S- 1 chemotherapy with no adverse events, such as a decrease in the CD4lymphocyte count or an increase in the HIV-RNA level. This patient with gastric cancer and HIV infection was safely treated using both antiretroviral therapy and chemotherapy owing to treatment intervention by chemotherapy and infectious diseases specialists.