A magnetic disk tester has a head clamp (35, 218) to position a magnetic head (15, 289). The head clamp has a micromotion stage (55, 273) provided with a stage (59, 205) which is horizontally moved by a piezo-element (77). The magnetic head is attached to the piezo-stage (55, 273). On the head clamp (218), a reflective scale (222) is attached to a back face of the piezo-stage, and a laser head (217) is oriented toward the head clamp (218). According to a reflected laser beam from the reflective scale, the position of the magnetic head (289) is detected. An error signal indicating the difference between the detected position and a reference position is used to control the head clamp (218). The piezo-element can operate at high frequencies, to easily make the magnetic head trace a target track and improve the positioning accuracy of the magnetic head.