PURPOSE: To make it possible to select a laser device which has the most suitable wavelength for the desired treatment, by measuring and displaying the difference in intensity of reflection with regard to many wavelengths from the nornal skin and the diseased skin before the application of the treatment using a nevus treating laser device.

CONSTITUTION: In a wavelength measuring device 42, a light beam, which has a wavelength specified by a wavelength selecting device 47, is selected and extracted out of light beams from a light source 41, and guided to a body to be measured A through a light guide 43. The reflected light from the body to be measured A is guided to a reflected light measuring device 44 through the light guide 43, and the intensity of the reflected light is measured. When the instruction for starting measurement is imparted to a controller 46 from a starting button 45, the wavelengths to be selected are sequentially spcified to the wavelength selecting device 47. Each measured value is memorized. When the measurement starting instruction is outputted again, the same procedure as the first time is performed. The difference between the measured value of the first time and that of the second time is computed and displayed on a display part 48.