Abstract: A focus adjusting plate 10 serving as a reference for focus adjustment is attached to a glass plane of a document stand 11, and a lens carriage 19 in a reading unit 1 is made movable minutely in a sub-scanning direction. The lens carriage is moved one pitch by one pitch within a minute movement range to read black stripes and white areas of the focus adjusting plate 10. The position of the lens carriage providing a maximum contrast is obtained, and the lens carriage 19 is fixed at the position. Thus, the optical path length L from the upper face of the glass plane of the document stand 11 to the lens 17 via mirrors 14, 15 and 16 is focus-adjusted. Accordingly, errors such as variations in accuracy in fabrication can be canceled to provide an image data in focus.