A white reference plate includes in addition to a rectangular portion disposed at an optical adjustment position, white reference portions between an optical adjustment position and a document size detection position outside a document reading region (i.e., outside contact glass) in a main scanning direction. In response to an opening and closing detecting section detecting opening and closing of a document conveyance unit during optical adjustment, a drive section moves a scanning section from the optical adjustment position to the document size detection position. The white reference portions are disposed at locations above respective opposite end parts of the scanning section in a main scanning direction, so that optical adjustment can be performed using an output value that the scanning section obtains by reading the white reference portions without interruption even during or after the movement.