Abstract: A rotating-probe type defect detecting device, used for continuous inspection of an elongated test object moving in the longitudinal direction, such as round metal bar drawn out from a set of dies, comprising a plurality of test probes supported by a rotating frame structure around the object in a plane orthogonal to the central axis of the object for revolving about the object at high rotational stability, is provided. The device includes an inventive circuit for processing the output signals of the probes to enhance defect indicative portions of the signals and suppress the noise level, thereby exhibiting an improved detection characteristic of high signal-to-noise (S/N) ratio.