A 68-year-old man with occult hepatitis B virus (HBV) infection was diagnosed with malignant lymphoma and achieved complete remission after treatment with a chemotherapy regimen including rituximab for 5 months. Entecavir (ETV) was also used during and after chemotherapy and was ended at 14 months after chemotherapy. However, reactivation of HBV was observed in blood tests, which showed not only elevation of HBV-DNA but also HBsAg and HBeAg, at 27 months after the end of chemotherapy. After restarting ETV, the HBV-DNA levels immediately subsided. In addition, anti-HBs became and remained positive at 31 months after chemotherapy. ETV was re-discontinued at 36 months after chemotherapy.