A document deciding apparatus includes: a light source; a light-quantity detection unit detecting a reference light quantity based on irradiation of the light source, and respective light quantities of plural colors based on reflection from or transmission through a document; a decision unit deciding whether the document is colorless or colored, in accordance with a predetermined criterion on the basis of the light quantities of plural colors detected by the light-quantity detection unit; and a control unit performing a control so as to alter the criterion of the decision unit or a value corresponding to the light quantities detected by the light-quantity detection unit, on the basis of a change of the reference light quantity detected by the light-quantity detection unit.