According to one embodiment, a document detecting apparatus includes a light-emitting element, a photoelectric conversion element, a light-emission controller and a detection processor. The light-emitting element projects light onto a detection position of a document. The light-emitting element is not used for reading an image of the document. The photoelectric conversion element converts light projected onto the detection position into an electric signal and outputs the electric signal. The light-emission controller changes the light quantity of the light emitted by the light-emitting element during a predetermined interval. The detection processor detects the presence or absence of the document at the detection position of the document based on whether a value of the output electric signal during the predetermined interval changes in correspondence to the change of the light quantity caused by the light-emission controller.