Image forming apparatus having original reading unit movable relative to image forming unit

An image forming apparatus, comprising:

• an image forming unit having a front surface and a backside surface facing each other in a first direction;
• an original reading unit disposed above the image forming unit;
• and
• a guide mechanism for moving and positioning the original reading unit in the first direction over the image forming unit between the backside surface of the image forming apparatus and the front surface of the image forming apparatus,
• wherein said guide mechanism includes a wall portion disposed above the image forming unit and extending in the first direction;
• a guide groove formed in the wall portion and extending in the first direction, said guide groove including a guide groove main portion extending in the first direction, a first positioning groove end portion formed at one end portion of the guide groove main portion, and a second positioning groove end portion formed at the other end portion of the guide groove main portion, said first positioning groove end portion being situated close to the backside surface relative to the second positioning groove end portion, said first positioning groove end portion being situated behind the backside surface in the first direction;
• and
• a protrusion protruding from a side of the original reading unit in a second direction perpendicular to the first direction for engaging the guide groove to be slidable.