We herein present a case of metachronous primary lung cancers, the first of which was adenosquamous cell carcinoma and the second of which was poorly differentiated adenocarcinoma. At the time the second lung cancer was detected 5 years after being operated on for the first cancer, a high level of serum alpha-fetoprotein (AFP) was noticed, but no elevation of other tumor markers was observed. In addition, no liver metastases, chronic liver diseases or other systemic abnormalities were seen either. The serum AFP level was 696 ng/ml, and the profile of lectin affinity showed a tumor-derived pattern. Two weeks after the operation, the serum AFP level decreased to a normal level. An immunohistochemical analysis confirmed that the exact origin of AFP was the tumor tissue. A specimen taken from the first lung cancer was not stained by the same procedures, which thus indicated this case to be a double primary lung cancer.