Although the etiology of classical trigeminal neuralgia is clearly understood to be neurovascular compression, the exact etiology of trigeminal neuralgia with continuous pain is often unknown. Mild sphenoid sinusitis is not usually considered to induce trigeminal neuralgia, especially when limited to the maxillary nerve. We report a rare case of trigeminal neuralgia of the maxillary nerve caused only by mild sphenoid sinusitis and discuss the significance of the anatomical structure and diagnostic procedures. A 45-year-old woman noticed a sudden onset of temporal pain followed by numbness on her right cheek. Her right gingiva also experienced sensory disturbance. The symptoms gradually subsided after the initial onset, but they persisted. She visited our hospital for further examinations and had no febrile episodes throughout the course. A tingling sensation and sensory disturbance were only identified in the maxillary nerve. No other neurological symptoms were noted. Magnetic resonance imaging revealed mild sphenoid sinusitis on the right side. The absence of the bony boundary between the sphenoid sinus and maxillary nerve was revealed using thin-sliced computed tomography (CT). The patient's symptoms were diagnosed as maxillary neuropathy caused by mild sinusitis. The bony defect around the maxillary nerve was considered to have affected development of the pathological process. Even mild sphenoid sinusitis can cause inflammation to spread to the maxillary nerve if no bony boundary exists between it and the sphenoid sinus. A coronal CT study is highly beneficial for clarifying the pathophysiological mechanism of trigeminal neuralgia limited to the maxillary nerve.