PURPOSE: To detect a foreign matter even when the foreign matter is transparent and wherever the foreign matter may be present in a conduit, by casting parallel beams on a transparent visible part set in the fluid conduit.
CONSTITUTION: A visible part V is set at a part of a fluid conduit 1 so as to be able to observe the interior of the conduit 1. Cylindrical glass parts 2 are smoothly connected at both ends of a rectangular glass part 3 at the visible part V. End parts of the cylindrical glass parts 2 are connected to the conduit 1. A collimator 5 turns a luminous flux from a light source 4 into parallel beams, and a photographic lens 6 is constituted of a group of lenses to make it possible to observe the whole area in the thicknesswise direction of a fluid in Z-axis direction and the whole visual field in Y-axis direction. A one- dimensional CCD sensor 7 scans the whole area of a mask slit 8 in the widthwise direction, with outputting a signal proportional to an intensity of the coming light to a signal-processing device 11. The signal-processing device 11 processes the signal and detects count, shape, size, etc., of foreign matter 10 in the fluid. Since the parallel beams are cast, a contrast becomes clear even when the foreign matter is transparent, making the detection of the foreign matter possible.