PURPOSE: To prevent mingling of scattering light with laser exciting fluorescence, by providing a pair of an introducing window and drawout window of a laser beam parallel to each other and a fluorescence takeout window being situated at right angles to said windows at a projecting part of an apparatus.

CONSTITUTION: An optical window for introducing a laser beam 3 and an optical window for drawing out the laser beam 3' are provided in parallel with an opposite position to an apparatus main body 1 and an optical window for laser beam exciting fluorescence takeout 3" is provided in parallel with a light passage of the laser beam. In case that a laser beam 4 is worst, laser scattering light 6, 6' are not emanated directly from the window 3" even when the beam 4 is made incident vertically to the end of the window 3 because the length of projecting parts 2, 2', 2" attaching the windows 3, 3', 3" is made larger than its diameter. Interfering scattering light is not mingled with a laser exciting fluorescence 5 because reflection on the wall face is prevented also by carrying out blackening treatment of the inner part of the apparatus.