PURPOSE: To take a high-precision of radial displacement characteristics of a displacement detector by cutting the 1st positioning groove on a circumference which has a specific radius centering on the center of rotation of a turntable and also cutting the 2nd positioning grooves partially on a radial straight line.

CONSTITUTION: The 1st V-shaped positioning groove 11c is cut on the circumference which has the radius R0 centering on the center 11b of rotation of the turntable 11 and the 2nd positioning grooves 11e and 11f are cut partially on the radial straight line 11d. The displacement detector 3 is constituted by attaching a measuring element 34 consisting of an electric micrometer to an XY stage 33 by micrometer heads 31 and 32 and the position of the measuring instrument 34 is adjusted finely in an up and down and a horizontal direction. The displacement detector 3 is adjusted by the 2nd positioning grooves 11e and 11f so that it runs through the center of rotation of a video disk 2, and also adjusted by the 1st positioning groove 11c so that its position detection signal CS shows a measured radius from the center of rotation of the disk 2 accurately, so the radial surface shape of the video disk 2 is reflected faithfully with high precision.