A 56-year-old man presented at our hospital with an abnormal shadow on his chest X-ray. He was diagnosed with fibrotic nonspecific interstitial pneumonia, and prednisolone and cyclosporin therapy were initiated after steroid pulse therapy. Before initiating an immunosuppressive agent, serological tests revealed that HBs antigen, anti-HBs antibody, and hepatitis B virus infection (HBV)-DNA were negative, but anti-HBc antibody was not tested. During the first 3 months after immunosuppressive therapy, his serum transaminase levels increased. Also, HBs antigen and HBV-DNA became positive, indicating the development of de novo hepatitis B virus infection (HBV) resulting from the reactivation of a resolved hepatitis B virus infection. On the fortieth day after admission, he died of hepatic failure in spite of intensive care. Intensive immunosuppressive therapy, such as rituximab therapy or stem cell transplant, is widely recognized as a contributer to the development of de novo hepatitis B virus infection. On the other hand, it has never been reported that conventional immunosuppressive therapy for idiopathic interstitial pneumonia may cause de novo hepatitis B virus infection. This case report indicates that de novo hepatitis B virus infection with fatal prognosis can be caused by conventional immunosuppressive therapy for idiopathic interstitial pneumonia. At the initiation of this therapy, it is important to detect the history of hepatitis B virus infection by checking patient's HBsAg, anti-HBs antibody, and anti-HBc antibody.