Ovarian tumor has been reported as the disease with abdominal enlargement in adult female carp. In this study, we report large tumor structure found in an adult male carp（8 years old). Diseased carp showed the abdominal enlargement, exophthalmus and scale protrusion. At necropsy, a large tumor（14cm), some small tumors（0.3-1cm）and abdominal ascites were found in body cavity. The hematocrit level and concentration of red blood cell are 9.5%, 8.2×10^8 cells/ml, respectively. Histopathologically, the large tumor was composed of parenchyma which was satisfied with propagated tumor cells and stroma which was spongiform connective tissue and fibrin.