A seven-year-old male, an intact miniature Schnauzer dog with history of vomiting, abdominal distention, anorexia, and dyspnea was referred for further evaluation and treatment. Thoracic radiographs showed the well marginated solitary mass with soft density in the right caudal lung field, and abdominal radiographs showed signs of ascites, such as abdominal distention and moderate serosal detail loss. On ultrasonograph and computed tomograph, it was observed that the mass compressed the caudal vena cava (CVC) and adhered to the heart. Exploratory thoracotomy was performed, and then it was showed that mass adhered heart, CVC, and diaphragm. The mass was fully resected although adhered part of CVC could not be completely resected. On histopathological findings, the mass was diagnosed as small-cell anaplastic carcinoma.