An automatic sheet conveying mechanism comprising an automatic document conveying mechanism and having a sheet guide plate (102) defining an opening (P9b) for receiving an elongate contact type image sensor (109) for scanning a lower surface of a document conveyed in a sheet conveying path (103) and guiding said lower surface of said document. The sheet guide plate (102) is integrally formed with an accommodating part (102c) for accommodating the image sensor (109) in a predetermined position, and the opening (P9b) is dimensioned to enable the image sensor to be put in and taken out of said accommodating part (102c) from above said sheet guide plate (102).