A plaque detecting device which determines the presence or absence of plaque on a tooth surface, comprising: a light emitting unit which irradiates ultraviolet or blue excitation light toward said tooth surface, and a first and second light receiving units (51, 52) which receive radiated light (L') from said tooth surface induced by said excitation light, characterized in that said first light receiving unit extracts, from said radiated light (L'), a spectral component of a first wavelength region having a predetermined lower limit wavelength and including the wavelength range of fluorescent light specific to plaque, and obtains a first output value corresponding to the intensity of the spectral component of this first wavelength region, and said second light receiving unit extracts, from said radiated light (L'), a spectral component of a second wavelength region having a predetermined lower limit wavelength lower than the lower limit wavelength of said first wavelength region and including the wavelength range of fluorescent light specific to enamel, and obtains a second output value corresponding to the intensity of the spectral component of this second wavelength region, the plaque detecting device further comprises a control unit configured to act as: - a first determination unit which performs determination of a relative magnitude of a ratio between said first output value and said second output value as compared to a predetermined first threshold value (a), and - a second determination unit which performs determination of a relative magnitude of a difference between said first output value and said second output value as compared to a predetermined second threshold value (β), wherein a substance present on a tooth surface is identified as being or not being plaque based on a combination of the determination results from the first determination unit and the determination results from the second determination unit. a light emitting unit which irradiates ultraviolet or blue excitation light toward said tooth surface, and a first and second light receiving units (51, 52) which receive radiated light (L') from said tooth surface induced by said excitation light, characterized in that said first light receiving unit extracts, from said radiated light (L'), a spectral component of a first wavelength region having a predetermined lower limit wavelength and including the wavelength range of fluorescent light specific to plaque, and obtains a first output value corresponding to the intensity of the spectral component of this first wavelength region, and said second light receiving unit extracts, from said radiated light (L'), a spectral component of a second wavelength region having a predetermined lower limit wavelength lower than the lower limit wavelength of said first wavelength region and including the wavelength range of fluorescent light specific to enamel, and obtains a second output value corresponding to the intensity of the spectral component of this second wavelength region, the plaque detecting device further comprises a control unit configured to act as: - a first determination unit which performs determination of a relative magnitude of a ratio between said first output value and said second output value as compared to a predetermined first threshold value (a), and - a second determination unit which performs determination of a relative magnitude of a difference between said first output value and said second output value as compared to a predetermined second threshold value (β), wherein a substance present on a tooth surface is identified as being or not being plaque based on a combination of the determination results from the first determination unit and the determination results from the second determination unit. - a first determination unit which performs determination of a relative magnitude of a ratio between said first output value and said second output value as compared to a predetermined first threshold value (a), and - a second determination unit which performs determination of a relative magnitude of a difference between said first output value and said second output value as compared to a predetermined second threshold value (β), wherein a substance present on a tooth surface is identified as being or not being plaque based on a combination of the determination results from the first determination unit and the determination results from the second determination unit.