PURPOSE: To prevent lowering of drilling positioning precision and to stabilize unevenness in drilling precision by preventing the occurrence of the warp and the sag of a printed wiring board during measurement through irradiation of an NC guide mark, displayed on the printed wiring board, with X-rays.

CONSTITUTION: An X-ray generator 5 to irradiate an NC guide mark 4 on a printed wiring board 3 and an image tube 7 irradiated with X-rays are provided. Annular clampers 13 clamped on the upper and under surfaces of the printed wiring board 3 and at the periphery of the NC guide mark 4 is attached to a working rod 12 of an air cylinder 11. By clamping the upper and under surfaces of the printed wiring board 3 by means of annular clampers 13 through the working of the air cylinder 11, the warp, the sag, and deformation of the printed wiring board 3 are corrected during measurement and the accurate position of the NC guide mark is measured.