A diffraction grating comprises a substrate with a set of protruding ridges and intervening trenches characterized by a ridge spacing \u039b, width d, and height h. The inclined sheet-separate plate is detachably attached to the back-surface support portions such that a back surface of the inclined sheet-separate plate is supported by the back-surface support portions. The data processing apparatus calculates a ratio of the extracted records to all records having the page of the page number and presumes whether the problem relates to a master object or relates to a variable object according to the ratio.