This was a case of a woman in her 60s with the chief complaint of an abnormal stomach X-ray at the screening. Although suspected to be scirrhous gastric cancer, gastric biopsy revealed Group 1, and cytology in accumulated ascites and open surgery was initially Class II, but cancer cells in the ascites were confirmed for the first time by subsequent immunostaining using the cell transfer technique. Undifferentiated advanced gastric cancer, peritoneal dissemination, and lymphatic metastasis were pathologically observed. This case suggests the effectiveness of immunostaining when the results of ascites cytology are different from the clinical picture.