An image forming apparatus includes a main body unit which has an image forming unit; a door unit which couples with the main body unit to open and close the main body unit, and has a projection at one side thereof; a mid-transfer unit which couples with the door unit; a coupling knob which is provided in the main body unit and rotates in cooperation with pressure of the projection when the door unit is closed; and a coupling apparatus which is provided in the main body and transmits a driving force from the main body unit to the mid-transfer unit in cooperation with the rotation of the coupling knob.