PURPOSE: To detect simply the original size by detecting the original size with an original size detecting sensor by having only to close an original retaining cover while an original is set to an original placing part.

CONSTITUTION: When an original retaining cover 4 is closed, a light of a light source 51 is given directly to a luminous energy detecting sensor 42 via a slit 21 and its luminous energy is measured. In this case, the luminous emergy of five positions of the light source 51 along the slit is measured. A picture signal in response to the content of the original is obtained from an image sensor by moving an original read main body 5 in the lengthwise direction of the original picture via a drive belt 7 of the motor 6 in this state. Thus, the original size is detected by an original size detecting sensor 41 assembled in the cover 4 by having only to close the original retaining cover 4 while the original is set on the original placing section 2 in this way.