PURPOSE: To correctly detect the degree of modification of the surface of an article by inducing the emission of fluorescence of the intensity inversely proportional to the degree of modification through irradiation of the surface of the surface of a substrate with ultraviolet rays.

CONSTITUTION: The ultraviolet ray irradiated from an ultraviolet ray irradiating means 12 is projected on a desired area 22 of the surface 16 of a substrate 14. The substrate 14 irradiates fluorescence of the intensity inversely proportional to the degree of modification by the component of a modifying agnet. The irradiated light forms an image on a photosensitive surface 24 of a detector 26 through a lens 23. A CCD, a cadmium sulfide photocell, a silicon photocell, etc., is used as the detector 26. A display means 32 is connected to a computer means 30 to display the output signal from the detector 26. For example, the display means 32 includes a cathode ray tube to display the image of the area 20. The analog display is also possible by displaying the level of the signal on a recorder or a voltmeter using a long paper in place of the cathode ray tube.