PURPOSE: To carry out image forming action without taking into account the original size by carrying out selection or magnification setting of a paper feeding cassette according to the detected original size.

CONSTITUTION: In an original size detecting part 33, the original 36 is set so that its base edge is abutted to a cursor 37 on an original platen glass 2, an original platen cover 3 is closed, and detection pulse of a yellow bar 33 is counted by a counter 38 from a position where a scanning point is off a leading tip of the original 36 by having it scanned by an optical sensor 35 afterwards. Therefore, by inputting a count value of the counter 38 to a size calculating part 39 of the next stage, the size data of the original 36, that is the data showing discrimination between an AB column system and an inch system and the size is obtained. Thus selection of a paper feeding cassette according to the detected original size and the setting of the magnification can be carried out. Thus, either of the AB column system or the inch system can be applied and operation convenience is improved.