Background: A new regimen of intravenous (IV) and intraperitoneal (IP) Paclitaxel (PTX) combined with oral S-1 is effective as the first line chemotherapy for gastric cancer with peritoneal metastasis. However, no information is available for the benefit of gastrectomy at salvage setting for the patients with clinical downstage. Methods: A total of 100 patients with peritoneal metastasis of gastric cancer received combination chemotherapy of PTX from both IV (50 mg/m²) and IP (20 mg/m²) routes using subcutaneous implanted peritoneal access ports at on days 1 and 8. S-1 was administered at 80 mg/m²/day for 14 consecutive days, followed by 7 days rest. Gastrectomy was performed in 60 patients who showed apparent shrinkage of peritoneal nodules as well as negative peritoneal cytology at 2^nd look laparoscopy. In 30 of the 60 patients with gastrectomy, mRNA of carcinoembryonic antigen (CEA) in peritoneal lavages obtained from the port were quantitatively evaluated by CEA-mRNA Index (CmRI: CEA mRNA/ PBGD mRNA×10⁴) at each chemotherapeutic cycle. Results: MST of the 100 patients was 23.5 months. MST and 1y-OS of the 60 patients with gastrectomy were 34.5 months and 83%, while those of the other 40 patients were 13.0 months and 39%, respectively. In all of the 30 gastrectomized patients, CmRI at the initial laparoscopy was considerably high (median=15368, 145-398179), which was fairly reduced after 3 courses of the combination chemotherapy before surgery (median=25, 0-10286, p=<0.001). Among them, 2y-OS of 22 patients whose CmRI was reduced to the value <200 after 3 course of chemotherapy was significantly better than that of the other 8 patients with CmRI >200 (76% vs 25% p=0.02), suggesting the volume of tumor cells in peritoneal cavity may be a critical determinant for postoperative outcome. Conclusions: Combination chemotherapy of S-1 plus IV and IP PTX followed by gastrectomy is a promising strategy for peritoneal metastasis of gastric cancer. Periodical measurement of CEA-mRNA levels in peritoneal cavity can be a useful biomarker to predict the clinical efficacy of conversion gastrectomy.