1. A magnetic disk drive comprising a waveform equalization means for equalizing waveforms reproduced from a magnetic disk, and an adaptive learning means for adaptively learning an equalizing characteristic of said waveform equalization means, wherein said magnetic disk drive further comprises an abnormal learning detection means for detecting an abnormal operation in said adaptive learning means just after learning of a sector when said adaptive learning means adaptively learns the equalizing characteristic of said waveform equalization means, and an equalizing characteristic resetting means for resetting the equalizing characteristic before adaptive learning to an initial value before learning of a next sector when said abnormal learning detection means detects abnormality in said adaptive learning means.