PURPOSE:To always output a stable sinusoidal wave free from distortion without receiving the effect of gap variation, by controlling the bias magnetic field to a magnetoresistance element by detecting the gap between a recording medi um and the magnetoresistance element. CONSTITUTION:The gap between a magnetoresistance element 3 and a recording medium 1 is detected by a gap sensor 7 containing a metal plate 8 present at the same position as the element 3 and forming electrostatic capacity in opposed relation to the medium magnetically recorded by a sinusoidal wave signal. Corresponding to this detection output, the bias magnetic field to the element 3 is controlled through a controller 6 and a magnetic field generating means 5. As a result, a stable sinusoidal wave signal corrected in distortion is always obtained through a measuring apparatus 4 without receiving the effect of gap variation.
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