PURPOSE: To shorten the time for measurement by writing a test signal by the writing signal which is linearly increased in amplitude in one round of the tracks of a magnetic disk and detecting an envelope signal and the peak value thereof from the reading-out signal thereof.

CONSTITUTION: The writing of the test signal between the index signals of the tracks is executed by the writing current which is linearly increased in the amplitude from a current sweep generator 3 by the control of a microprocessor 2. The envelope signal voltage Ee is detected from this reading- out signal by an envelope detector 5. Further, the peak value Ep is detected by a peak detector 6. The voltage kEp obtd. by multiplying the Ep by a factor k by a multiplier 7 and the voltage Ee are compared and a coincidence signal is outputted by a comparator 8 at the point of the time when both coincide. This coincidence signal is inputted to an adequate current value detector 9 and the current value of the generator 3 at the time of the input point is outputted as an adequate current value.