PURPOSE: To measure a floating amt. of a head slider with high accuracy and to measure a low floating head slider by adding a position detecting means of a relative position between a disk and the head slider, etc.

CONSTITUTION: A measuring optical system 10 is equipped with a laser light source 18, photodetectors 23 and 26, a condenser lens 27 and an objective lens 20. A means for changing a relative circumferential speed between a glass disk 11 and the head slider 21 into a step pattern continuously is equipped with a servo motor 12, a computer 16, a servo circuit 17 and a revolving speed detecting part 31. Moreover, this means is equipped with a position detecting part 36 for detecting a relative position of the head slider 21 to the glass disk 11 and a head feeding mechanism 32 for controlling this relative position, and the state of the head feed mechanism 32 is detected by the position detecting part 36, and position information is sent to the computer 16. A head mechanism 32 is controlled by the computer 16, and the head slider 21 is controlled to be in a desired position in the radial direction of the disk 11.