PURPOSE: To position the light through hole of a target in the center of a body to be centered speedily with high precision by using a holder which holds the target, a main body which supports it movably, and plural supports which expand and contact freely.

CONSTITUTION: The target 8 is supported by the target holder 9, which is supported by the main body 10. The main body 10 is provided with three supports 24 which contact the internal surface of the body 23 to be centered at about 120 equal angles. A sub-support 30 is rotated properly almost to the center position of the body 23 to be centered to adjust the length, arranging the centering device. Then, the distance from a surface 32 to be machined to the outer circumferential surface of the target holder 9 is measured by an inside micrometer 33 and a micrometer 14 fixed to the main body 10 is rotated on the basis of the measured value to move the target holder 9, positioning the light through hole 8a at an axial center position.