We report a rare case of cavitary lung metastasis of rectal cancer, diagnosed initially as septic pulmonary embolism. A 55- year-old woman underwent emergency Hartmann's operation for perforation of the rectal cancer with multiple liver metastases. A 2 cm-sized thin-walled cavitary lesion was seen in the left upperlobe of the lung by CT, and septic pulmonary embolism was suspected. She recoverd from sepsis after intensive care treatment. Pathological diagnosis is adenocarcinoma (tub2), T3N1M1, Stage Ⅳ, she underwent chemotherapy. Serum CEA level was high preoperatively but gradually decreased to normal 4 months after the operation. Multiple liver metastases showed calcification, and the lung lesions remained unchanged on CT. She continued chemotherapy while changing the anticancer drug due to side effects. One year and 5 months after operation, lung CT showed thickened wall and spicula around the cavitary lesion. Serum CEA level was normal, SLX and NSE slightly increased and serum aspergillus antigen was positive. Bronchial lavage cytology was Class Ⅰ and scrape cytology was Class Ⅲ in bronchoscopy. Lung metastasis, primary lung cancer or aspergilloma were suspected and we performed partial lung resection. The pathological diagnosis was rectal cancer lung metastasis.