Metastatic cancer cells originating from gastric cancer are detected by a method comprising the step of collecting a biological sample from a subject, the step of detecting the presence of at least either aldehyde dehydrogenase or dopa decarboxylase in the biological sample of the subject, and the step of determining that the possibility of containing metastatic cancer cells originating from the gastric cancer in the sample is high when at least either aldehyde dehydrogenase or dopa decarboxylase is present. By the use of these as markers for metastatic cancer cells originating from gastric cancer, the presence or absence of peritoneal metastasis in a gastric cancer patient can be detected rapidly and reliably, and data important for deciding whether intraperitoneal cancer chemotherapy should be applied is provided.