Signet ring cell carcinoma (SRCC), a subtype of adenocarcinoma, is observed usually as the primer cancers of stomach, large intestine, bladder, prostate and breast. SRCC which has poor prognosis is rarely occured as primary lung cancer and the vast majority of SRCC found in the pulmonary system is metastatic. In making the diagnosis of SRCC in the lung, it is very important to exclude metastatic SRCC from the stomach, colon, bladder, prostate and breast. The main feature of this type of malignancy is mucin-producing cells. We reported this case for to point out the importance of differential diagnosis of primary and metastatic tumors, since they require different clinical approaches.