Provided is a sensitizing detection agent of an oral or intravenous administration type which enables the detection of bladder cancer with a higher sensitivity without causing pain to the patient. A sensitizing detection agent for bladder cancer comprising 5-aminolevulinic acid (ALA), a derivative thereof, or a salt of these is orally or intravenously administered, and a video camera system is inserted via the urethra and a blue light at 380-440 nm is irradiated to observe the red fluorescent part. Further, VLD-M1 is inserted and a blue light at 405 nm is irradiated to observe fluorescence intensity (relative intensity) of the red light part. For oral administration, 20 mg/kg (maximum of 1 g) of ALA is dissolved in 50 mL of a 5% glucose solution prior to the administration.