A 54-year-old man complaing of epigastralgia and pyrosis since September 2001 was referred to the hospital because upper gastrointestinal endoscopy revealed a huge tumor on the antrum at another hospital. Physical examination revealed gynecomastia, and blood test showed high level of serum HCG, 13, 400. Upper GI series and gastrointestinal endoscopy visualized a huge gastric tumor with ulcer formation located from the posterior wall to the greater curvature of the antrum. Abdominal CT scan demonstrated extra-gastric extension, multiple lymph nodes metastasis and multiple liver metastasis. He was diagnosed as having primary gastric choriocarcinoma and was not a candidate for a curative operation, so chemotherapy with TS-1 was started. But the multiple liver metastasis rapidly grew and the patient died on January 5, 2002. Choriocarcinoma can present gynecomastia because chorionic gonadotrophin is elevated in blood. It is highly malignant and curative resection is often impossible when it is detected. HCG-producing tumor must be kept in mind as a differential diagnosis in treating patients who have gynecomastia of unknown origin.