A 40-year-old man complaining of voiding symptoms was referred to our hospital for further examinations and treatment of giant bladder diverticulum and possibly underlying neurogenic bladder dysfunction. Because computed tomography and magnetic resonance imaging revealed the presence of a tumorous lesion covering the bladder diverticulum, transurethral biopsy was performed to elucidate its pathological characteristics. Through the histopathological examination of the biopsy specimens the tumorous lesion was diagnosed as ganglioneuroma which was benign. The whole diverticulum with the concomitant tumor lesion was surgically excised. Full examination of the surgical specimen concluded the final histopathological diagnosis of the tumor to be plexiform neurofibroma. As multiple café-au-lait spots were found on the patient's skin, he was clinically diagnosed with neurofibromatosis type 1.