Of all oral squamous cell carcinomas, squamous cell carcinomas of the buccal mucosa (BSCC) are most associated with poorest prognosis. In particular, patients treated with surgery alone are reported to experience frequent local failures. This is considered to result from the surgeon's determination of the deep surgical margin for resection based on palpation alone when performing BSCC surgery. Therefore, an objective system for classifying the depth of invasion of a tumor appears to be necessary in order to improve the results of BSCC treatment. While current general practice is to treat based on tumor thickness, we would like to emphasize the importance of how far down the cheek wall layer the tumor has invaded. We performed surgery by classifying the depths of tumor invasion in relation to the buccinator. Depth of tumor invasion was assessed mainly using ultrasonography (US). The tumor was defined as D1 when it extended to the mucosal (m) and submucosal layers (sm). In these cases, the tumor was resected, while the buccinator was spared. The tumor was defined as D2 when it extended to the buccinator, but, based on US, muscle continuity was preserved, and the tumor was resected to include the buccinator and its overlying fascia. When the tumor had spread to the buccinator or invaded subcutaneous or cutaneous tissue it was classified as D3 and resection included the skin. The disease-specific survival rate of BSCC when treated based on our classification was 73.7% and the local control rate was 89.5%. These results are superior to those based on surgery alone and this therapeutic modality was considered to be useful.