a document reading apparatus comprises a contact type image sensor having a protection glass located at a surface where a document is to be read , first roller mechanism for supplying a document which is to be read in a document transport direction on the protection glass of the contact type image sensor , a platen roller which is rotatably supported at a position confronting the protection glass , a support mechanism for rotatably supporting the platen roller , and a restricting mechanism . the support mechanism permits a movement of the platen roller in a predetermined direction which is approximately perpendicular to a surface of the protection glass depending on a thickness of the document which is supplied between the platen roller and the protection glass by the first roller mechanism . the restricting mechanism restricts a position of the platen roller to a predetermined position when no document is inserted between the platen roller and the protection glass so that a predetermined gap is formed therebetween .