PROBLEM TO BE SOLVED: To precisely judge the size of an original without the effect of environment factors by judging the size of original based on the combination of a comparing result concerning a precisely obtained before shielding representative value and that concerning after shielding representative value. SOLUTION: A control part 54 controls the position of a carriage member so that the sensing area of a CCD line sensor 44 is at a prescribed position. Next, the control part 54 compares picture information at each small area obtained before a transparent member is shielded by a cover and a first threshold value with each other. Next, the control part 54 compares a representative value at each small area obtained after the transparent member is shielded by the cover and a threshold value set after the transparent member is shielded with each other. Then based on the combination of the comparing result by a comparing means at each small area, a size detection part 58 judges the size of the original on the transparent member.