A sheet with a thickness .DELTA.x is attached on a surface of a plate glass in an image reading area, against which a subject-sheet is pressed, so that an air layer of thickness .DELTA.x can be formed between the surface of the plate glass and a surface of the subject-sheet at the opposite side. The thickness .DELTA.x is determined to reduce the effects of an optical interference of the illumination light from a light emitting diode. The sheet has an uneven surface in order to decrease an area where the sheet is in contact with the subject-sheet, thus reducing the adhesion between the sheet and the subject-sheet. A member with a low coefficient of friction is used as the sheet.