A 71-year-old male was admitted for abdominal fullness. The condition rapidly deteriorated in a short period (3 weeks), and the patient died. Autopsy revealed a protruding lesion measuring about 3 cm with erosion measuring 5 mm in diameter immediately below the esophago-gastric conjugation site, suggesting primary cardial undifferentiated adenocarcinoma. In the primary focus, changes on the mucosal surface were almost normal However, below the mucosa, infiltration of cancer cells was observed in an approximately 3 cm area along the gastric wall. Simultaneously, the site of infiltration was markedly increased in deep areas. Extraserous infiltration was observed. The morphology was special, and resembled an octopus pot, a trap used to catch octopuses in Japan, with a narrow top and a broad base. In our patient, metastatic lesions were detected in multiple organs, including the stomach.