PURPOSE: To shorten the waiting time for the operator during copying and to enhance the manipulatability by detecting a sheet size of a document before the start of copying operation so as to equalize the times of copying for a first sheet in both document sheet size detecting mode and document sheet size nondetecting mode to each other.

CONSTITUTION: When the operator opens a document cover 35 for setting a document D, an actuator 64 is elevated so as to turn off a cover switch 63 so as to release a condition in which first and second sensors 60a, 60b are inhibited from detecting a document sheet size. When the document D is set on a platen 34, a light receiving element 62 receives light which is emitted from a light emitting element 61 and which is reflected by the document D in each of the sensors 60a, 60b, that is, the script D is detected. Further, the sensor 60b delivers an input signal indicating that a document is present, to processors so that a scanning motor 41 is at once driven, and accordingly, a first carriage 38a starts scanning for detecting a sheet size of the document D. During this scanning, the output levels of the sensors 60a, 60b are determined by the processors. After completion of the detection of this sheet size of the document, the detection of the sheet size of a document is deferred once.