A wavelength of light emitted from a semiconductor laser is shifted to a shorter wavelength with wavelength converting means and the resulting light of a shorter wavelength is applied to a sample. Upon exposure to the light of the shorter wavelength, the sample emits light of interest and its waveform is measured with measuring means. Fundamental wavelength laser light which passes through the waveform converting means is outputted therefrom in synchronism with the light of a shorter wavelength and detected by a first photodetector. The waveform of the light of interest can be measured correctly on the basis of an output signal from the first photodetector.