Image forming apparatus

1. An image forming apparatus comprising:

an image forming unit that forms an image on a sheet;

a driving roller that is provided on a downstream side of the image forming unit and rotates in forward and reverse directions;

a first driven roller that is driven to rotate and forms a first nip portion in cooperation with the driving roller;

a second driven roller that is driven to rotate and forms a second nip portion in cooperation with the driving roller, the second driven roller being provided on a downstream side of the first nip portion;

a first transfer passage into which the sheet passed through the first nip portion and sent from the second nip portion is transferred by a forward rotation of the driving roller;

a second transfer passage into which the sheet passed through the first nip portion is transferred by the forward rotation of the driving roller without passing through the second nip portion; and

a third transfer passage into which the sheet passed through the first nip portion and nipped at the second nip portion is transferred by a reverse rotation of the driving roller after a rear end of the sheet has passed through the first nip portion by the forward rotation of the driving roller.

2. The image forming apparatus according to claim 1 , wherein the third transfer passage is formed as a re-transfer passage for retransferring the sheet on one surface of which the image has formed by the image forming unit to the image forming unit again with the sheet being upside-down.

3. The image forming apparatus according to claim 1 , wherein the first transfer passage is configured to transfer the sheet on one surface of which the image has formed by the image forming unit to a first tray on which the sheet is stacked with an image-formed surface thereof directed downward; and

wherein the second transfer passage is configured to transfer the sheet on one surface of which the image has formed by the image forming unit to a second tray on which the sheet is stacked with the image-formed surface thereof directed upward.

4. The image forming apparatus according to claim 1 , further comprising a transfer passage shifting member provided between the first nip portion and the second nip portion around the driving roller, the transfer passage shifting member being displaceable to a forward direction guide position at which the sheet sent from the first nip portion is guided to the second nip portion and a reverse direction guide position at which the sheet sent from the second nip portion is guided to the third transfer passage.

5. The image forming apparatus according to claim 1 , further comprising a cover that covers a side portion of the image forming apparatus on the downstream side of the first nip portion, the cover capable of being opened and closed and capable of stacking thereon the sheet transferred along the second transfer passage when the cover is opened.

6. The image forming apparatus according to claim 5 , wherein the second driven roller is attached to the cover.

7. The image forming apparatus according to claim 1 , wherein the image forming unit comprises:

a heating roller that rotates while heating the sheet; and

a pressure roller that forms a fixing nip portion in cooperation with the heating roller and urges the sheet passing through the fixing nip portion against the heating roller; and

wherein the image forming apparatus further comprising an intermediate driven roller provided between the fixing nip portion and the first nip portion and on an opposite side of the first driven roller with respect to the sheet, the intermediate driven roller being driven to rotate while being in contact with the sheet in a position projected from a common tangent line of the heating roller and first driven roller toward the first driven roller than.

8. The image forming apparatus according to claim 1 , wherein the image forming unit comprises:

a heating roller that rotates while heating the sheet;

a pressure roller that forms a fixing nip portion in cooperation with the heating roller and urges the sheet passing through the fixing nip portion against the heating roller; and

a fixing unit having a case body covering the heating roller and the pressure roller; and

wherein the case body comprises a chute configured to guide the sheet transferred along the third transfer passage.