A plaque detecting device is provided with: a light emitting unit (450) which radiates ultraviolet or blue exciting light (L) toward a tooth surface (99a); and first and second light receiving units (402) which receive emitted light (L') from the tooth surface (99a). The first light receiving unit extracts from the emitted light (L') a first wavelength region spectrum component containing a wavelength range of fluorescence unique to plaque, and obtains a first output value corresponding to the intensity of the first wavelength region spectrum component. The second light receiving unit extractsfrom the emitted light (L') a second wavelength region spectrum component containing a wavelength range of fluorescence unique to enamelum. The second wavelength region has a lower limit wavelength that has been set in advance to be lower than the lower limit wavelength of the first wavelength region. The magnitude of an interval between a ratio and a first threshold is determined, with the ratiobeing a ratio between the first output value and the second output value. A difference between the first output value and the second output value is obtained, and the magnitude of an interval betweenthis difference and a second threshold is determined.