A first positioning member is provided on a document table to extend in a first scanning direction and determines a position in a second scanning direction of an edge of a document placed on the placing surface. A first reference area holding member extends from a first positioning member over the placing surface and holds, on its surface opposing the placing surface, two reference areas extending parallel to the first scanning direction and including a first and second reference areas each having a different pattern from each other. A border position in the first scanning direction between at least one of the first and second reference areas and the document is detected, based on a received light quantity of reflected light. A size determining unit calculates a document length in the first scanning direction based on the detected border position.