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This paper presents a method for measuring the head slider spacing fluctuation induced by wavy deformation in a disk. The author's first experiment reveals that disk wavy deformation, which is considered to appear in building up a magnetic disk drive, shortens working life time. To supply a reliable disk drive, it is necessary to evaluate disk waviness after building up a disk drive. The simple and practical readback signal modulation (RSM) method was investigated here. The second experiments, including the measurement of the spacing loss effect in recording, reveal that spacing fluctuation, measured by the RSM method, is in good agreement with that estimated theoretically from the disk displacement waveform. It becomes possible to evaluate the head slider spacing fluctuation quantitatively after building up a disk drive.