We aimed to define the role of cyclophosphamide, vincristine, doxorubicin, prednisone (CHOP) followed by involved field radiotherapy (IFRT) for treating localized primary gastric diffuse large B-cell lymphoma (DLBCL). Newly diagnosed patients with localized primary gastric DLBCL were to receive four cycles of CHOP followed by IFRT of 40.0 Gy. At 1 year after the completion of treatment, patients filled out the EORTC Quality of Life Questionnaire specified for stomach cancer (QLQ-C30-STO22). Fifty evaluable patients (25 men, 25 women) were included. The median age was 54.5 years (range, 21 - 73 years. The overall response rate to the CHOP was 94% (95% confidence interval [CI], 87 - 100) in the intent-to-treat population. Forty-one of the 50 patients (82%; 95% CI, 71 - 93) achieved complete remission (CR). After the completion of radiotherapy, five patients who were in PR following chemotherapy eventually attained CR. The overall complete response rate was thus 92% (95% confidence interval, 84 - 99). With a median follow-up period of 30 months, the 2-year progression-free and overall survival rate was 92% and 92%, respectively. The gastric function was well preserved with negligible stomach-related symptoms at 1 year after the completion of treatment. This organ-preserving combined treatment is highly effective and well tolerated for the patients with localized gastric DLBCL.