PURPOSE: To make a scan on the outer circumference of a cylinder without providing a track by providing individually clamping means for fixing a couple of clinging members which cling to the cylinder individually to the cylinder.

CONSTITUTION: An arm 5b is closed to allow a rear ring 10, front ring 11, and scanner ring 12 to cling to the cylinder 27. Then, the ring 11 is fixed to the cylinder 27 by a clamp pad 13b. At this time, the rings 10 and 12 are in a free state, so the ring 10 is moved under the guidance of a guide rod 16 by rotating a feed screw 15. Then, the ring 10 is fixed to the cylinder 17 by a clamp pad 13a, and the screw 15 is rotated while the ring 11 is free to move the rings 11 and 12. Thus, the ring 12 is moved to make an axial scan on the cylinder 27, and a scanner 21 arranged at the ring 12 is swiveled to scan a sensor 22 in the circumferential direction of the cylinder 27.