A 28-year-old woman visited a local hospital with a complaint of progressive dyspnea after a normal delivery. She was given a diagnosis of COP based on TBLB specimen and CT findings and received steroid pulse therapy at the hospital. Three months after her first visit she was referred to our hospital because her symptoms had not resolved despite receiving steroid therapy. The CT findings at our hospital suggested pulmonary thromboembolism. Although anticoagulation therapy was initiated, the CT findings and symptoms worsened. Thus thrombi were removed by open chest surgery and a diagnosis of choriocarcinoma was established. No primary lesion was found on gynecological examinations. Complete remission was achieved after systemic chemotherapy and there have been no event in the two years since diagnosis. Choriocarcinoma is a rare malignant tumor developing after pregnancy, with frequent metastases to the lung. However, when the tumors embolize into the pulmonary arteries, they resemble pulmonary thromboembolisms making correct diagnosis difficult. Since, owing to recent advances of cancer chemotherapy, choriocarcinoma is now curable tumor, physicians must suspect this disease in cases of pulmonary embolisms in women.