Abstract: In a foreign matter detecting apparatus 1, light beams are applied to a moving plastic resin extruded from an extruder, and a foreign matter, if any, in the plastic resin is detected by transmitted light beams. First and second projectors 30a,30b of the foreign matter detecting apparatus apply a plurality of light beams with different convergence diameters to the plastic resin which is extruded and moves in a passage 18. First and second light receivers 40a,40b receive the light beams transmitted through the plastic resin, and compare their respective reception levels with a plurality of levels of light reception which are previously set corresponding to the size of the foreign matter. Based on the result of comparison, the light receivers detect the presence and size of the foreign matter involved in the plastic resin.