PURPOSE: To reduce the size of the flaw detecting device and to reduce its S/N ratio by passing a wire rope through a magnetizing device and magnetizing it, detecting leak magnetic flux from a defect of the rope by a differential type magnetic sensor, and inputting its magnetic signal to a slicing circuit.

CONSTITUTION: When the wire rope is passed through the magnetizing device 2. The magnetizing device 2 consists of a circular permanent magnet 4 and a cylindrical yoke 5. Further, the differential type magnetic sensor 3 is provided around the wire rope 1. Then, the rope 1 is magnetized almost to its saturation point and if the rope 1 has a defect, leak magnetic flux is generated and detected by the sensor 3. A voltage signal from the sensor 3 is inputted to the slicing circuit and a DC servo circuit to remove a noise signal, amplifying the flaw signal. Thus, the rope is magnetized by the permanent magnet, so the device is reduced in size and defects the rope are detected continuously with less noises.