A first photosensor is sensitive to the wavelength of excitation light, insensitive to the wavelength of fluorescent light, and receives a portion of the output light to generate a first current corresponding to the amount of light received. A second photosensor is sensitive to the fluorescent light wavelength, insensitive to excitation light wavelength, and receives a portion of the output light to generate a second current corresponding to the received light amount. A first current/voltage conversion circuit outputs a first detection voltage corresponding to the voltage drop across a first resistor. A second current/voltage conversion circuit outputs a second detection voltage corresponding to the voltage drop across a second resistor. If (i) a relation between the magnitudes of the first detection voltage and the second detection voltage has reversed, or (ii) if the first detection voltage deviates from a normal voltage range, a judgment unit asserts an abnormality detection signal.