A woman in her early 60s noticed bilateral breast masses and visited a different hospital. Core needle biopsy showed diffuse large B-cell lymphoma of the right breast and invasive ductal carcinoma of the left breast. After referral to our department, PET-CT was performed. Compared with mild fluorodeoxyglucose accumulation in left breast cancer(BC), highly accumulated lesions were found on the right breast, left anterior chest wall, nasopharynx, and tonsil. The right breast lesion was the largest with a diameter of 30mm and was considered the primary lesion of malignant lymphoma(ML). The ML was classified as stage Ⅳ, pathologically proven with erythema of the left breast and nasopharynx. Three courses of R-CHOP were performed. However, due to suspicion of heart failure, chemotherapy was changed to R-CEOP(non-anthracycline-containing regimen)and 3 courses were additionally performed. The therapeutic effect of R-Chemo for ML was CR. Left BC showed a tendency of shrinkage. After intrathecal administration of anticancer drugs to prevent infiltration of ML into the central nervous system and preoperative endocrine therapy with aromatase inhibitor, left lumpectomy and sentinel lymph node biopsy were performed. BC was classified as clinical stage ⅠA and had an estrogen receptor score of 3b. Postoperative whole breast radiotherapy was completed, and the planned internal use of exemestane was more than 5 years. With multidisciplinary therapy, 3.5 years had passed since the initial treatment without recurrence.