To provide a foreign matter detecting device which makes a foreign matter being desirable to be not removed but left, difficult to detect, even in the case a threshold for detecting a foreign matter being desirable to be removed is set severe in detecting foreign matters mixed in a yarn, i.e., which prevents the detection accuracy for the foreign matter being desirable to be removed from lowering, even in the case of avoiding the foreign matter being desirable to be left from being detected.

The foreign matter detecting device 32 alternately irradiates a spun yarn 9 formed of cotton fibers with light (direct light Da, Db) from LEDs 37a, 37b and measures intensity of reflected light Rb, Ra from the spun yarn 9 to detect presence of the foreign matter mixed in the spun yarn 9 on the basis of the intensity information. The light source color of the LEDs 37a, 37b is yellow or yellowish orange, i.e., in a hue ring, the light source color is the same as or similar to the color of trash of stems or leaves of cotton fibers which is the foreign matter being desirable to be not removed but left.

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