A 33-month-old indoor sow showed a sudden loss of appetite and then died. Necropsy revealed a sponge-like appearance of the liver parenchyma, encephalomalacia, and dark red coloration of the heart. Histologically, extensive necrotic lesions were detected in the liver, brain and heart, and Grampositive rods were detected in these necrotic lesions. Immunohistochemically, the rods reacted with an antibody against Clostridium species. Anaerobic cultures yielded high numbers of Clostridium novyi (C. novyi) type B. These findings suggested that necrosis and encephalomalacia were associated with C. novyi type B. C. novyi type B infection was diagnosed as the cause of death, and this was a case of fatal C. novyi type B infection with gas gangrene in an indoor sow.