A 63-year-old man with decompensated liver cirrhosis and pure red cell aplasia complained of pyrexia, abdominal distention and abdominal pain. A diagnosis of spontaneous bacterial peritonitis (SBP), Conn's syndrome, was made upon the isolation of an anaerobe Clostridium perfringens from both ascitic fluid and peripheral blood. The bacteria were found to be susceptible to piperacillin, and administration of the antimicrobial agent markedly improved his SBP. The anaerobes should be kept in mind as one of the possible pathogens of SBP, although anaerobic infection has been reported to be quite rare in the disease.