PURPOSE: To detect the munber of projections having varying heights without changing the number of revolutions of a floating head or a magnetic disc by providing a detecting element for elastic waves generated by collision against the projections to the floating head, and comparing the outputs thereof with the set thresholds of an amplitude discriminator.

CONSTITUTION: A detecting element which detects elastic waves generated by collision against projections is provided to a floating head 1, and the outputs thereof are inputted to an amplitude discriminator. The signal waveforms of the detecting elements are as shown by Figure (b), and the peak values thereof change according to the extent of floating of the head 1 and the heights of projections 3W6. The peak values are beforehand prepared as data, and threshold values are set as shown by A, B, C of a figure (b). For the detection signals exceeding said values, pulses are formed like cWe, and are counted. Therefore the number of the projections differing in height is detected simply by changing the threshold values without changing the number of revolutions of the floating head or the magnetic disc.