PURPOSE: To enable a higher efficiency in directional adjustment of incident X rays, by arranging two metal fine wires mounted along the diameter of a chi-ring and a goniometer suspended along the diameter of the chi-ring.

CONSTITUTION: A point of a needle 2 so mounted on a goniometer head 3 as to let the tip thereof at the position of a sample is on a rotating shaft of a chi-ring. Metal fine wires 6 are mounted with a mounting device 5 on the ring 1. The fine wires 6 are mounted so as to pass separately through the rotating shaft of the ring 1 and consequently, the intersection of the two fine wires makes a point on the rotating shaft of the ring 1. Under such a condition, X rays are incident from one side of the ring 1 to check an incident state thereof downstream by photography. As a positional difference between the intersection of the fine wires 6 and the needle point 2 in a photograph causes a vertical deviation in the incident X rays. Thus, deviation of the incident X rays from the rotating shaft of the ring 1 can be learned by taking into account of distance to the fine wires 6 from the needle point 2 to make a pint to be corrected in optical systems clear thereby enabling efficient implementation of the centering in an incident direction.