A scanning device is provided with a first transparent plate on which an original sheet to is placed, the original sheet being scanned statically, a reading unit arranged below the first transparent plate and configured to scan an image on the original sheet in the main scanning direction, the reading unit being movable in an auxiliary scanning direction, and a first reference member defining a reference position for scanning an image on the original sheet, in the main scanning direction and in the auxiliary scanning direction. The first reference member is arranged at an unused area of the first transparent plate, the unused area being an area which is within the original sheet placeable area on the first transparent plate and is not covered with original sheets of a plurality of predetermined different sizes placed on the first transparent plate with being positioned with respect to a predetermined positioning point.