A method for sensitively diagnosing lower urinary tract disorders, particularly interstitial cystitis, is provided. The method can visually, simply, and clearly diagnose bladder ulcer without requirement of bladder hydrodistention under anesthesia. In the method, lower urinary tract disorders are diagnosed by observing an abnormality of a blood vessel and/or a newly formed blood vessel in a surface of bladder mucous membrane among abnormalities in blood vessels and/or newly formed blood vessels by visually comparing an image of the surface of the bladder mucous membrane and an image of a deep portion of the bladder mucous membrane obtained using a bladder endoscope system having a narrow band imaging device.