PURPOSE: To always obtain an accurate read signal wave amplitude mean value by calculating the read signal wave amplitude mean value based on an integration value of maximum peak values for a period from the moment of the polarity of each of two read signals inverted from the negative polarity into the positive polarity till the moment of the polarity of each of two read signals inverted from the positive polarity into the negative polarity divided by number of times of inversion from the positive into the negative polarity.

CONSTITUTION: An amplifier section 2 amplifies two read signals 31, 41 with a phase difference of 180° from a magnetic head 1 and outputs the amplified signals to wave amplitude mean value calculators 10, 20. Then each of the wave amplitude mean value calculators 10, 20 receives respectively one of the two read signals and calculates the read signal wave amplitude mean value based on an integration value of maximum peak values for a period from the moment of the polarity of each of two read signals inverted from the negative polarity into the positive polarity till the moment of the polarity of each of two read signals inverted from the positive polarity into the negative polarity divided by number of times of inversion from the positive into the negative polarity. Moreover, an adder 3 calculates the read signal wave amplitude mean value from both the calculation values of the 1st and 2nd wave amplitude mean value calculators 10, 20. Thus, the read signal wave amplitude mean value calculators 10, 20. Thus, the read signal wave amplitude mean value is measured accurately at all times.