PURPOSE: To enable the realtime measurement of foreign matters in a fluoric acid solution, by measuring a number of foreign matters simultaneously.

CONSTITUTION: Optical fiber bundles 16 and 17 are covered with a cover 18 made of a fluorocarbon resin immune to fluoric acid. For light to move into or out of end faces 16' and 17' of the fiber bundles 16 and 17, the thickness of the fluorocarbon resin is minimized at the end faces thereof. The fiber bundle 16 thus obtained is placed into a fluoric acid solution and when an irradiation light is transmitted thereto, it is made to radiate an area 19 through the end face 16' with no loss. On the other hand, the fiber bundle 17 having the end face 17' is placed into the fluoric acid solution with the end face 17' set near the end face 16'. As a result, a light receiving area of the end face 17' is limited to an area 20, which makes scattered light or the like reach the end face 17', as caused by foreign matters generated at an area 21 as defined when the areas 20 and 19 cross. Thus, the fiber bundle 17 can detect foreign matters in the solution. Since the quantity of light such as scattered light increases in proportion to the number of the foreign matters in the area 21, the number of foreign matters in the solution can be determined immediately by calibrating the relationship between the quantity of light and the number of foreign matters preliminarily.