PURPOSE: To secure an accurate detection for the number and the size of the microprojections by detecting the ultrasonic wave caused through the contact between the magnetic head and the microprojection on the disk via the ultrasonic wave detection element mounted on the magnetic head and then comparing the output value with the reference value through the comparator.

CONSTITUTION: The ultrasonic wave caused by the contact between the magnetic head and the microprojection on the disk is detected throuth piezoelectric element 1 functioning as the ultrasonic wave detection element. This output is then made to pass through band pass filter 3 via preamplifier 2 to obtain the ultrasonic wave vibration waveform as shown in Fig. (a). This waveform is compared with the fixed reference level sent from level setting circuit 5 at analog comparator 4, and the rise coincidence signal is applied to monostable multivibrator 6. The trigger pulse duration of multivibrator 6 is set to T+δ at time constant circuit 7 as shown in Fig. (b), and the retriggering is given when the amplitude of the waveform of Fig. (a) is higher than the reference level to obtain the pulse of Fig. (c). Based on the number of output pulse 8, the number of the microprojections can be obtained, and then the pulse duration is measured by the fixed clock. And the size is obtained by comparing the reference level with the data prepared to the size of the known microprojection.