An image reading device capable of correctly reading an image on a document is provided without the need to increase the size of an upper cover, etc. of the device. A CIS (Contact Image Sensor) is supported by holders and support shafts to be movable toward the upper cover (rib) and is biased toward the rib by compression springs. In this configuration, the rib for holding the document is not required to be installed as a movable component and there is no need to provide a space for allowing the rib to move or providing the upper cover with a mechanism for allowing the rib to move, which can avoid the need to increase the size of the upper cover.