In this study, intra-peritoneal chemotherapy was performed for advanced gastric cancer with serosal invasion. We assessed the efficacy of the chemotherapy using quantitative genetic diagnosis with peritoneal lavages collected before and after chemotherapy at staging laparoscopy and/or gastrectomy. Cases in which genetic diagnosis of peritoneal lavages showed negative results, or changed from positive to negative after chemotherapy, had a good prognosis. On the other hand, cases in which genetic diagnosis showed positive results after chemotherapy had worse prognosis. Intra-peritoneal chemotherapy may be indicated for patients with a positive genetic diagnosis only and cases becoming negative as a result of this therapy may have a better prognosis.