PURPOSE: To shorten the time for measurement by writing a test signal which is linearly increased in frequency in one round of the tracks of a magnetic disk and correcting the crest values of the envelope signal of the reading-out signal thereof by a modulation correcting factor.

CONSTITUTION: The writing current which is linearly increased in the frequency is outputted from a frequency sweep generator 5 by the command of microprocessor 2 and the test signal is written in one round of the tracks. The crest values to the respective rotating angles of the envelope signal detected by an envelope detector 7 are similarly transferred to the processor 2 and are multiplied by a modulation factor, by which the data of the normal envelope signal to the respective rotating angles is obtd. Since the relations between the rotating angles and the frequencies are previously known in the processor 2, the frequency D50 which has the amplitude of half the low frequency is determined from this detector and is outputted to an output section 2b.