A dental-plaque detection system and dental-plaque detection method which are excellent in the detection accuracy of dental plaques and also capable of improving the situation to stain sites other than dental plaques (such as interdental parts and gums) are provided. The dental-plaque detection system includes a dental-plaque staining agent, which contains at least one selected from yellow pigment of beni-koji, turmeric extracts, and curcumin; and a light-emitting apparatus, which outputs light having a wavelength within a range of 250 to 500 nm to an object in the oral cavity where the dental-plaque staining agent is attached. The dental-plaque detection method includes attaching a dental-plaque staining agent, which contains at least one selected from yellow pigment of beni-koji, turmeric extracts, and curcumin onto an object in the oral cavity; and thereafter irradiating light having a wavelength within a range of 250 to 500 nm onto the object.