We encountered a patient who had a primary unknown carcinoma with lumbago caused by lumbar vertebral metastasis as the initial chief complaint. Autopsy revealed primary pulmonary adenocarcinoma. Abnormal findings noted on plain chest radiogram and chest CT led to misdiagnosis as inflammatory changes. The primary lesion was poorly differentiated pulmonary adenocarcinoma that could not be diagnosed correctly using the diagnostic criteria of these diagnostic modalities.