PURPOSE: To detect exactly an original size by delaying an exciting pulse supplied to a light source by prescribed time, a synchronizing this delayed pulse and an output pulse by the exciting light from a phosphor, and cutting noise mixed into the output pulse.

CONSTITUTION: A pulse generating circuit 22 is connected to a delaying circuit 24, and a delay pulse V4 delayed by a prescribed time Td from an exciting pulse V1 is generated. In a part where an original 14 exists, near infrared ray emitted from a light emitting element 16 is reflected by a platen glass 12 and the original 14, and reaches a filter 17. In such a case, the filter 17 does not make the infrared ray transmit through, therefore, a photodetector 18, etc. go to a high level, and other part goes to a low level. An irregularly mixed noise is almost eliminated and an input voltage V5 to a controlling circuit 23 goes to a low level. A noise mixed into a output pulse is cut, therefore, the adverse influence caused by noise, disturbance, etc. is prevented, and an original size can be discriminated exactly.