A medium supply unit of an image forming apparatus, the medium supply unit may include: a main body housing; a tray connected to the main body housing and loading a printing-target medium therein; an activating shaft connected to the main body housing and being rotated in a predetermined direction by an activating force of an activating source; an intermediate member supported by one end of the activating shaft to deliver a rotating force of the activating shaft; a first elastic member having one end connected to the intermediate member and the other end supported by the activating shaft; and elastically biasing the intermediate member in a direction facing the printing-target medium according to a rotation of the activating shaft; and a roller unit having a roller unit housing that is attachable to the intermediate member and at least one roller that is supported by the roller unit housing and picks up and supplies printing-target media loaded in the tray; and configured to deliver to the roller(s) an elastic force of the first elastic member and a rotating force of the activating shaft when the activating shaft is rotated.