PURPOSE: To test a magnetic disk by the same circuit even when a test frequency is different by rectifying a readout signal and comparing the result with a specific inspection level, inputting the output signal by said comparing means on the basis of a pulse signal, and deciding on whether each magnetic disk is normal or not.

CONSTITUTION: When the read of a defective track is executed, the signal level from an amplifying circuit 4 and a rectifying circuit 6 drops greatly. Consequently, a counter 20 counts up the pulse signal from a PLO7. Then, when the read returns to the normal state, the output signal level of the amplifying circuit 4 and rectifying circuit 6 becomes sufficient and the counter 20 stops counting operation. When a defective track is detected again, the contents of the counter 20 are stored in a memory 20 successively by the same operation. When the reading is completed by repeating said operation, the contents of the memory 24 are checked to know the number and size of defective track parts. Consequently, even when the test frequency is different, the same circuit performs the inspection without being altered.