INTRODUCTION: In recent years, patients with orthognathic surgery in middle-aged and elderly people have come to be a more frequent occurrence. Breast cancer is the most frequently diagnosed cancer in woman worldwide, and its prevalence rate is steadily increasing. PRESENTATION OF CASE: We report a case of a 47-year-old Japanese woman in whom left-side breast cancer (Stage 1) was unexpectedly found just before orthognathic surgery in April 2012. Breast-conserving surgery was performed (estrogen receptor+, progesterone receptor+, HER2 -, surgical margin+, sentinel lymph node +) that May. From June to August docetaxel (75mg/m2) and cyclophosphamide (600mg/m2) were administrated four times every 21days and thereafter radiotherapy (total 60Gy) was completed. The cancer surgeon declared the prognosis good and the patient had a strong desire to undergo orthognathic surgery, so in November we performed a bimaxillary osteotomy, and administration of tamoxifen began 6 weeks after the osteotomy. DISCUSSION: There are breast cancer cases in which the prognosis is sufficiently good for a planned orthognathic surgery to proceed. Good communication among surgeons and the patient is important. CONCLUSION: We experienced a case in which breast cancer was found just before the orthognathic surgery; we performed a bimaxillary osteotomy, including follow-up tamoxifen administration, during breast cancer treatment.