To obtain a quantity of light which makes a dynamic range larger in a original to be read by performing alignment of an image scanning means and a light source for transparent original reading, based on luminous intensity distribution that is actually measured in an image reader which synchronously scans with an image scanning means and a light source for reading a transparent original and reads a transparent original.

A relative table between a vertical scan luminous intensity distribution and a reading position is produced by stopping a light source L2 for transparent original irradiation at a certain position on an original mounted glass 101 with light on and reading the vertical scan direction luminous intensity distribution at a body before reading a parent original. At the time of reading an original, correction is performed so as to align a read position of a body image scanning means from a detected relative table. Thus, with a light source of light quantity which makes a dynamic range larger the original is read.