In an apparatus at a spinning preparatory plant for detecting foreign objects, for example, pieces of cloth, tapes, string, pieces of sheeting and the like, in fibre material, the fibre material is transportable in an air flow through a fibre transport duct or a feed chute and an optical sensor system is associated with the duct or chute, the wall surfaces of which have at least one transparent region through which the sensor system detects the fibre-air flow. To permit the at least one transparent region to be kept clean in a simple manner during operation, and to permit an unobstructed detection of the foreign objects, the transparent region projects into the fibre-air flow and the fibre-air flow is able to flow along the transparent region in force-applying contact therewith.