To estimate true coordinates for a guide mark, with the effect of altitude removed out of a guide mark X-ray image formed on a multilayer printed circuit board conductor layer.

A guide mark M formed in a conductor layer in a multilayer printed circuit board 60 is irradiated with X-rays emitted radially and linearly from an X-ray source A which is virtually a point. An image Z is formed on a fluorescent plane of an X-ray camera positioned directly under the guide mark M, and an x-coordinate c1 and a y-coordinate d1 are determined for the image Z. Relative to the X-ray camera, the wiring board is moved by L1 in the x-direction and by L2 in the y-direction for the guide mark M to move to the position M2. Then an x-coordinate c2 and a y-coordinate d2 are determined for the image Z2 of the guide mark M2. The coordinates (Lx, Ly) estimated for the guide mark M, which are calculated by using guide mark coordinates estimation formulas [Lx=L1×c1/(c2-c1), Ly=L2×d1/(d2-d1)] in reference to (c), (d), are not affected by the distance between the guide mark M and the X-ray camera fluorescent plane.