The representative configuration of an image reading unit according to the present invention and an image forming apparatus comprising the same is an image reading unit comprising a document feeding unit, a lamp and a mirror for reading out the image on a document, a flow reading platen provided between the document feeding unit, and the lamp and the mirror for guiding the document while sliding the same against the surface, and an aluminum sheet for grounding the flow reading platen, wherein the aluminum sheet is conductive with the flow reading platen on the document feeding direction upstream side with respect to the document reading position for reading the image in the substantially entire document width by the lamp and the mirror.