To obtain a defective screw part detecting machine which removes the dent etc., in thread ridges of screw parts generated when passing through processes of heat treatment, plating, etc., and on the other hand, performs discrimination of defective units by detecting the run out in a vertical or lateral direction of the screw parts.

The machine is equipped with flat thread gauges 2, 3 on stationary and moving sides which are arranged to face each other and when a screw part A moves rotationally between both gages 2 and 3, the thread ridges are corrected by form rolling in the outer peripheral part of the screw part A. On the other hand, both the gauges 2, 3 are provided with sensors 7, 8 for measuring the angle at which a base of a head A fits to the threaded part of the screw part A, and the discrimination of the non-defective unit and the defective unit is performed based on the result of the detection by the sensors 7, 8. Regarding the non-defective unit, a defective unit expulsion passage 14 for expelling the defective unit separately from the non-defective unit is provided.