PURPOSE: To measure the surface and internal characteristic of a running plate material simultaneously, by inputting continuous X rays to the surface of the running plate material, and detecting and analyzing the transmitted X rays, which are emitted in a straight line, and the transmitted and diffracted X rays, which are emitted at the same angle as the incident angle to the side of the surface normal.

CONSTITUTION: X rays 22A are continuously inputted to the surface of a running plate material 10 at a specified incident angle θ with respect to the surface normal B. The X rays 24A, which are transmitted straight through the running plate material 10, are detected by a transmitted X-ray detector 24. The difference between the amount of the transmitted X rays and the amount of the incident X rays 22A is corrected. Thus the thickness of the running plate material 10 is measured. Transmitted and diffracted X rays 26A are emitted at the same angle θ as the incident angle θ on the same side of the incident X rays 22A with respect to the surface normal B of the running plate material 10. The X rays 26A are detected by a transmitted- and diffracted-X-ray detector 26. The intensity of energy is analyzed and the result is corrected by the attenuation amount of the transmitted X rays 24A when the plate thickness is measured. In this way, the surface and the internal characteristics of the running plate material are measured. Thus on-line measurement can be performed.