A flatbed scanner includes a housing, a glass plate fixed to the housing, a front rib formed at a bottom face of a front part of the housing and extending downward, a back rib formed at a bottom face of the back part of the housing and extending downward, and side ribs formed at two sides of the housing and extending toward the center of the housing. The inner distance from the back rib to the front rib is not smaller than the length of the glass plate. The side ribs are under the bottom faces of the front part and the back part so as to form a fastening gap. Because the distance between the side ribs is smaller than the width of the glass plate, when the glass plate is inserted into the fastening gap, the glass plate is held.