An amplitude measurement unit measures an amplitude of a read signal from a preamble unit in a servo area of a predetermined frame number m per one rotation of the recording medium for a predetermined cycle number n while increasing an electric power amount distributed to the heater in the state in which the head is positioned to a track on the recording medium, and a mean amplitude calculating unit calculates a mean amplitude from an amplitude measurement value which is measured by the number which is the frame number m multiplied by the cycle number n. A contact determination unit calculates a mean amplitude variation amount per a unit heater electric power amount as a contact determination value, and determines that the head is in contact with the recording medium when the contact determination value is lower than a predetermined threshold value, and a flying height calculating unit which calculates a flying height of the head with respect to the surface of the recording medium from the heater electric power amount at the point when contact is determined.