Magnetic disk device for detecting a floating state of a magnetic head

Gain information in a read amplifier, which is recorded when data to be measured is reproduced, is stored in an AGC control memory circuit in advance. By reproducing the data using the gain information, reproduced data varied according to the amount of flotation of a magnetic head and the position of the magnetic head alone can be obtained. A variation in voltage amplitude of the reproduced data is detected by an envelope signal generating circuit and a variation in phase difference between a reading cycle in which the reproduced data is read and a reference cycle is detected by a phase difference signal generating circuit. A calculator calculates a float interval of the magnetic head and a variation in the position thereof based on these detected variations. As a result, a decision can be made as to whether the float interval of the magnetic head and the variation in the position thereof exceed an allowable range. When it is judged that they have exceeded the allowable range, a warning is issued to a user so that the magnetic head and a magnetic disk can be prevented from breakage in advance.