Reading apparatus with compact drive unit assembly

A reading apparatus includes an auto document feeder configured to be openable and closable around a hinge with respect to a unit including an image sensor. The auto document feeder includes a roller configured to move a document relative to the image sensor, a motor, a gear train configured to transmit a rotation of a rotational shaft of the motor to the roller, and an encoder unit configured to detect rotation information of the motor, and including a sensor and a code wheel. An orientation of the rotational shaft is substantially parallel to a rotational axis of the hinge, and the gear train is arrayed in a direction of an extension of the rotational shaft. A gear that is a part of the gear train is attached to one end of the rotational shaft, and the code wheel is attached to the other end thereof.