An automatic document feeder that is included in an image reader of an image forming apparatus includes a sheet conveying roller, a document feeder cover, a projection piece, a motor, a driving side gear, and a driving force transmission device including an idler gear, a driving force release gear, a slide body, a first elastic body to bias the idler gear and the driving force release gear to be separated, a second elastic body to bias the driving force release gear and the slide body to be separated, and a drive shaft. The driving force transmission device meshes the driving and driven side meshing portions together with a biasing force applied by the second elastic body when the document feeder cover is closed, and releases the driving and driven side meshing portions with a biasing force applied by the first elastic body when the document feeder cover is opened.