PURPOSE: To observe precisely the surface state of a body heated to high-temperature by irradiating the surface of said body with two kinds of monochromatic light pulse consisting of a fundamental wave and its higher hamonics, and passing through the reflected light through an interference filter and a high-speed optical switch.

CONSTITUTION: When the flame 3 of the high-temperature heating body, e.g. the frame of burner is observed by picking up its image, two kinds of monochromatic light pulse having a 1.06μm wavelength and a 0.53μm wavelength are generated by an YAG laser 1 with a higher-harmonic generator and projected upon the flame 3 of the burner. Its reflected light and light emitted by the flame 3 are superposed with one another, passed through a half-mirror 5, and then passed through the interference filter 6 having transmission characteristic to permit to pass through only said two wavelengths to remove the light emitted by the flame; and the pulse light passed through a polarizer 7 and the high-speed optical switch 8 reaches an image generating mechanism 10 through an analyzer 9 and converted into an image to recognize visually the surface state of the flame 3 while background light is removed.