A housing body includes a first part forming a part of a first transporting path in which a medium is transported. An image forming unit is accommodated in the housing body and is adapted to form an image onto at least one face of the medium. A door cover is attached to the housing body, and the door cover includes a second part adapted to face the first part when the door cover is closed, thereby forming the part of the first transporting path together with the first part. A double-sided printing unit is detachably mounted to the second part and includes a second transporting path. The second transporting path is connected to the first transporting path and is adapted to transport the medium while turning inside out, so that the image forming unit forms images on both sides of the medium.