A strategic surgical approach is necessary for patients who cannot undergo maxillary sinus augmentation due to a large perforation of the sinus membrane as a result of complex sinus septa. The technique includes partial cutting and removal of the sinus septum and graftless mucosal elevation of the concavity area. Six months after the procedure, bone growth was observed in the area where the septum had been partially removed. The sinus mucosa was slightly thicker because of scarring; consequently, maxillary sinus augmentation was safely achieved. This two-step procedure is effective for safe maxillary sinus augmentation in patients with complex sinus morphology.