PURPOSE: To select automatically a form to facilitate the copying work by operating an original size detector, where plural photosensors are provided on a moving body constituting a scanner, for a certain period when movement of the scanner is started.

CONSTITUTION: Four photosensors AWD which detect both ends of an original Pa of size A3 and an original Pb of size B4 are provided on a moving body 9a along a reference line Y orthogonal to a scanning direction X. Photosensors E and F which detect end parts in the transverse direction of an original Pd of size A4 and end parts in the longitudinal direction of the original Pb of size B4 are provided on a moving body 9b along the center line in the scanning direction X. Detection signals of an original size due to photosensors AWF in a certain period after the start of operation of a scanner 5 are sent to a controller. A form size corresponding to the original size is determined by the controller, and the form is supplied from a form supply cassette. Thus, the copying work is facilitated.