The combination chemotherapy with cyclophosphamide, doxorubicin, vincristine, and prednisolone (CHOP) plus rituximab is the standard treatment for patients with primary gastric diffuse large B cell lymphoma (DLBCL). However, a few trials comparing CHOP plus rituximab (R-CHOP) with CHOP have been conducted in primary gastric DLBCL. Among 93 consecutive patients receiving CHOP or R-CHOP as a first-line chemotherapy at our institution, 38 patients received CHOP and 55 patients received R-CHOP. With a median follow-up time of 48 months, the complete response (CR) rate, event-free survival (EFS), and overall survival (OS) did not differ between two treatment groups (P = 1.000, P = 0.744, and P = 0.213, respectively). The CR rates were 93.9% for patients receiving CHOP and 92.5% for patients receiving R-CHOP. The 3-year EFS rates were 86.0% for patients receiving CHOP and 81.7% for patients receiving R-CHOP; the 3-year OS rates were 94.7 and 84.7%, respectively. In a multivariate analysis, The CR rate was affected by the number of extranodal involvements (P = 0.011). The EFS and OS rates were affected by the Lugano stage (P = 0.067 and P = 0.008, respectively). High serum level of β₂-microglobulin was associated with worse EFS and OS in patients receiving R-CHOP (P = 0.018 and P = 0.015, respectively). In conclusion, the addition of rituximab was not found to have an impact on patients' outcomes with primary gastric DLBCL. The β₂-microglobulin in primary gastric DLBCL might be able to discriminate the patients' prognosis who are treated with R-CHOP chemotherapy.