PURPOSE: To enable correct measurement of a chemical state of chromium by a method wherein the surface of a chromate-treated film is ground in vacuum mechanically and then an X-ray photoelectron spectroscopy is applied to the ground surface without bringing this surface into contact with the atmosphere.

CONSTITUTION: In a grinding apparatus 1, a magnetic fluid seal 6 is provided for keeping the inside of the apparatus in high vacuum. A rotary blade 3 is connected to and driven by an external motor 7 through the seal 6. The position of the rotary blade 3 is fixed and mechanical grinding in vacuum is executed by moving vertically an iron stub 2, a sample, which is fixed to a stage holder 4 being movable vertically by a stage elevating mechanism 5, and thereby the clean surface of a chromate film is exposed without changing the chemical state of the film. An ordinary measuring apparatus of X-ray photoelectron spectroscopy being employed, the apparatus 1 is connected directly to a measuring chamber thereof and a strength ratio between trivalent chromium and hexavalent chromium in the chromate-treated film is measured without bringing the ground surface into contact with the atmosphere.