PURPOSE: To enable accurate measurement with the correction of variations in sensitivity by covering a part of a fluorescent detecting surface of a glass element o make an exposed area correspond to sensitivity of the glass element.

CONSTITUTION: In a holder 2, a plate body made of metal or a synthetic resin is formed into a rectangular parallelo-piped of 16×13×45mm and a rectangular ultraviolet ray incident window 22 is formed on a top face 21 while a rectangular fluorescence detection window is formed on a side 23. The perimeter of the fluorescence detection window 24 is so arranged to be a frame-shaped cover part covering a glass element 1. In this case, the glass element in a low sensitivity lot can be made smaller in area of the covered part at the fluorescence detection window of the holder to enhance the sensitivity with an increase in the exposed area of the fluorescence detecting surface. On the other hand, the glass element in a high sensitivity lot can be made larger in the area of the covered part to decrease the sensitivity with a reduction in the exposed part of the fluorescence detecting surface.