Sarcoidosis is an idiopathic systemic disease with non-caseating granuloma that is reported to occur concomitantly with various malignant tumors. We report here a case of advanced gastric cancer with sarcoidosis, for which precise preoperative diagnosis was difficult. A 69-year-old woman was diagnosed with a far-advanced gastric cancer because multiple intra-abdominal and intra-thoracic lymph node swellings and a tumorous lesion in the spleen were detected by multidetector computed tomography and fluorodeoxyglucose positron emission tomography. However, these multiple changes might be derived from sarcoidosis, because the multiple lymph node swellings were nonspecific changes for gastric cancer. With appropriate informed consent, surgery was selected in order to accurately confirm the stage of progression. Consequently, these changes were shown to not be metastatic, and curative surgery was performed. In conclusion, careful examination and a reasonable treatment strategy are important for avoiding the risk of overdiagnosis in gastric cancer patients with sarcoidosis.