Tocilizumab, an anti-human interleukin 6 receptor (IL-6R) monoclonal antibody, is widely used to treat rheumatoid arthritis (RA) and is expected to exhibit clinical efficacy when used to treat other autoimmune diseases. However, a risk of opportunistic infection is occasionally recognized. A 54-year-old woman had received an oral corticosteroid and methotrexate to treat RA. Despite receiving these treatments, she received additional treatment with tocilizumab due to poor control of the disease activity. She presented at our hospital with a high fever and epigastralgia 19 days after receiving this treatment. A laboratory evaluation revealed liver injury and cytomegalovirus (CMV) viremia. Abdominal ultrasonography and computed tomography (CT) revealed hepatosplenomegaly, but no ascites. Upper gastrointestinal endoscopy revealed gastric erosions induced by CMV, which were confirmed immunohistochemically. Hence, we diagnosed the patient with CMV reactivation-induced acute hepatitis and gastric erosions under tocilizumab treatment. She received an anti-cytomegalovirus drug, ganciclovir, for 14 days due to her viremia and impaired general condition, which was suggestive of a severe infection. Her general condition subsequently improved, the liver function test results normalized, and the gastric erosions disappeared. In conclusion, although tocilizumab is very useful for treating certain autoimmune and inflammatory diseases, and will be prescribed more widely in the future, associated CMV infections must be closely monitored, as these can be lethal.