There are provided an original table for an original, a linear image sensor for converting light into an electric signal, a first optical system for forming an optical image on a main scanning line onto the linear image sensor, a sub scanning unit for scanning the main scanning line in parallel with a sub scanning direction, and a second optical system forming an optical path that reaches the linear image sensor from a detecting position on the plate surface, which is remote from the main scanning line in a sub scanning direction. A size and an orientation of an original are decided based on an electric signal converted from light that reaches the linear image sensor from the main scanning line via the first optical system, and an electric signal converted from light that reaches the linear image sensor from the detecting position via the second optical system.