To provide an image reader capable of obtaining white reference data from which an influence due to dust or the like is removed without eliminating variations in individual elements of a CCD.

Image data obtained by reading a shading plate is stored in an image data storage unit. An average calculation unit calculates an average value of values of a predetermined number of pixels before and after a pixel of interest. Thereafter, a determination unit determines whether the absolute value of a difference between the average value and a value of the pixel of interest falls within a threshold. When the difference between the average value and the value of the pixel of interest falls within the threshold, a data correction unit outputs the value of the pixel of interest to a white reference data storage unit. When the difference between the average value and the value of the pixel of interest exceeds the threshold, if the value of the pixel of interest is larger than the average value, a value obtained by adding the threshold to the average value is output to the data correction unit. If the value of the pixel of interest is smaller than the average value, a value obtained by subtracting the threshold from the average value is output to the data correction unit.