1. A document reading apparatus comprising: a document reading portion including a document table having a document placement surface, a light source for exposing a document placed on the document placement surface to light, an imaging element for receiving light from the document exposed by the light source, and an imaging optical system for focusing the light from the document onto the imaging element to thereby form an image thereon, which imaging optical system has an imaging lens and a group of mirrors which moves in a sub-scanning direction relative to the document table and leads the light from the document to the imaging lens; a first chart portion having a predetermined reference chart, which is disposed at a predetermined first position on the document table; a second chart portion having the predetermined reference chart, which is disposed at a predetermined second position spaced away in the sub-scanning direction from the predetermined first position on the document table; and an image processing portion that obtains, based on respective chart images of the first and second chart portions which chart images are obtained from the predetermined reference charts read by the document reading portion, respective correction amounts of a plurality of image areas arranged in the sub-scanning direction in a document image of the document read by the document reading portion, and then based on the respective correction amounts, processes the document image.