OPTICAL DETECTING DEVICE

PURPOSE:To detect the concentration of fluid to be detected precisely without any influence of foreign matter by providing a hollow member which spreads outward to an intake, and guiding the fluid to a cylinder body and inhibiting the entry of foreign matter. CONSTITUTION:A fluid admitting member 70 consisting of a rod 72 and cylinder bodies 73 and 74 is provided to the front end part of the cylinder part 80 of a transmitted light type detector S having a light emitting element 32, a photodetecting element 42, etc., across a ring 71 so that it projects from the intake P in a spread shape. Consequently, forward fluid to be detected, e.g. a dust flow is inhibited from entering the cylinder body 80 by the relation between the mutual intervals of the rod 72 and cylinder parts 73 and 74 and their surface shapes, and a lateral powdery dust flow due to a parallel running vehicle, etc., on the other hand, is inhibited by the relation between intervals of the cylinder part 74 and intake P, and cylinder part 74 and ring 71, and the outer surface shape of the cylinder part 74, so that only a dust flow after waste is removed is admitted into the cylinder body 80.