We investigated the development of hepatits B virus (HBV) reactivation in patients receiving immunosuppressive therapy or chemotherapy at our hospital for 8 years. Using the automatic checking system for HBV reactivation coded using medical information that has been in operation in our hospital since October 2012, we prospectively observed the occurrence status of HBV reactivation in immunosuppressive/chemotherapy cases for 8 years. HBV reactivation occurred in 31 of 1516 patients with HBV infection. It occurred annually between 1 and 7 cases in multiple clinical departments, and in 8 of 59 patients treated with rituximab, 10 of 653 patients treated with antineoplastic agents, 10 of 399 patients treated with steroids, and 3 of 212 patients treated with direct-acting antivirals. The cumulative incidence of HBV reactivation was 1.2%, 2.3%, and 3.4% at 1, 2, and 3 years, respectively. The results of Cox regression analysis showed that the incidence of HBV reactivation was significantly higher in patients who received rituximab (odds ratio:12.841) or steroid (hazard ratio:4.264) or those who tested positive for HBc antibody alone (hazard ratio:11.005). We observed the occurrence of HBV reactivation in HBV-infected patients treated with immunosuppressive therapy or chemotherapy. HBV reactivation by immunosuppressive therapy or chemotherapy still occurs, and further safety management and caution are required in the hospital.