To provide an image reader that can properly set an object value of a background level by each line on an original with a simple processing system, without increasing the data processing amount to enhance the reliability for the background eliminating processing.

On the basis of a peak level of an original background detected by a background detection area provided to the main scanning line, the background is traced so as to bring a background level output to a prescribed target white level output, but since there exists a difference between the peak level of the background detection area and the peak level of the entire scanning line, tracking to the target white level output causes deviations, then the target white level output is corrected depending on the deviation to attain proper tracking. A correction data processing section 39 uses a white reference board, before reading the original area to read the background detection area and the entire area to calculate the correction value as a ratio of peak levels detected by peak detectors 1, 2, the correction value is reflected on the gain of a gain processing section 41, together with the background correction value of a background elimination processing section 40 to trace the proper background.