A processing unit (12) reads out, from a memory (10), wavelength setting data corresponding to wavelength information from a key input section (9), and outputs a wavelength setting command corresponding to the wavelength setting data to a tunable-wavelength control section (13) and a spectral wavelength control section (14). The tunable-wavelength control section (13) controls a tunable-wavelength light source section (1) to output a laser beam having a wavelength corresponding to the wavelength setting data. The spectral wavelength control section (14) controls a spectroscope section (6) to perform spectroscopy of the light having the wavelength corresponding to the wavelength setting data and output the resultant light. Output light from the tunable-wavelength light source section (1) whose wavelength has been set is input to an object (15) to be measured. Light passing through the object (15) is input to the spectroscope section (6). The spectroscope section (6) performs spectroscopy of only the light having the wavelength corresponding to the wavelength setting data. A light-receiving section (7) receives the spectral light and photoelectrically converts it. The resultant light is converted into digital data by an A/D conversion section (8) to be input as level data to a processing unit (12). The processing unit (12) obtains a light-reception level on the basis of the digital level data.