Background: Irinotecan is a key drug in second- or further-line chemotherapy for patients with advanced gastric cancer. Continuous administration of trastuzumab beyond first progression is expected to contribute to the benefit of chemotherapy for HER2-positive gastric cancer. We assessed the efficacy and safety of combination chemotherapy with trastuzumab and irinotecan in Japanese patients with advanced HER2-positive chemo-refractory gastric cancer. Methods: Intravenous infusion of irinotecan every 2 weeks at a dose of 150 mg/m²; intravenous infusion of trastuzumab at a dose of 8 mg/kg on day 1 of the first cycle, followed by 6 mg/kg every 3 weeks. Administration of irinotecan and trastuzumab were repeated in independent schedules. The primary endpoint was disease control rate. The secondary endpoints were adverse events, response rate, time-to-treatment failure, progression-free survival, overall survival, and response rate stratified by prior trastuzumab use. This study was conducted by the Osaka Gastrointestinal Cancer Chemotherapy Study Group (OGSG). Results: From October 2012 to Augst 2014, 30 patients were enrolled and one patient withdrew before study treatment. Accordingly, 29 patients were assessable for efficacy and safety. The disease control rate was 65.5% [95% C.I. 45.7 - 82.1%], and the response rate was 20.7% [95% C.I. 8.0 - 39.7%]. The median progression free survival and the median overall survival were 3.7 and 7.5 months, respectively. The major grade 3/4 toxic effects were neutropenia (24%); anemia (24%); leucopenia (21%); anorexia (11%); fatigue (14%); hypoalbuminemia (24%); and hypokalemia (14%). One death (NOS) was considered to be related to the study. Conclusions: The results of combination Trastuzumab with irinotecan showed feasible and promising efficacy against advanced HER2-positive chemo-refractory gastric cancer. These findings indicated that trastuzumab continuation use might be beneficial. Clinical trial information: 000008626.