1. An image forming apparatus comprising: an image reading unit that includes a document transporting device that transports a document loaded in a loading position to a reading position, a loaded-document detecting unit that detects whether the document has been loaded in the loading position, a document table on which the document to be read is placed, a document holder that can be closed to cover an upper surface of the document table and can be opened to make a space above the document table, has a white back face facing the document table, and holds the document placed on the document table, and a placed-document detecting unit that detects whether the document has been placed on the document table, the image reading unit reading an image of at least one of the document loaded in the document transporting device or the document placed on the document table; an image forming unit that forms an image based on the image read by the image reading unit; an acceptance unit that accepts a request for the image forming unit to perform image formation; a paper transporting unit that transports a paper sheet on which the image is to be formed to the image forming unit; a mode selection unit that can select at least one of a first mode in which the paper transporting unit starts transporting a paper sheet when the image reading unit starts reading the image and a second mode in which the paper transporting unit starts transporting a paper sheet after the image reading unit finishes reading the image; and a control unit that, when the acceptance unit accepts an image formation request, controls the mode selection unit to select the second mode and controls the image reading unit to read an image on the document table if the loaded-document detecting unit detects that the document has not been loaded in the loading position and the placed-document detecting unit detects that the document has not been placed on the document table.